Diadophis punctatus, commonly known as the ringneck snake or ring-necked snake, is a species of colubrid snake. It is found throughout much of the United States, central Mexico, and south eastern Canada. Ring-necked snakes are secretive, nocturnal snakes that are rarely seen during the day time. They are slightly venomous but their non-aggressive nature and small rear-facing fangs pose little threat to humans who wish to handle them. They are best known for their unique defense posture of curling up their tails exposing their bright red-orange posterior, ventral surface when threatened.
Dorsal coloration is solid olive, brown, bluish gray to black, broken only by a distinct yellow, red, or yellow-orange neck band. Head coloration tends to be slightly darker than the rest of the body with tendencies to be blacker than grey or olive. Ventrally the snakes exhibit a yellow-orange to red coloration broken by crescent shaped black spots along the margins
Ring-necked snakes use a combination of constriction and envenomation to secure their prey. The snakes do not have a true venom gland, but they do have an analogous structure called the Duvernoy’s gland derived from the same tissue.
Ring-necked snakes first strike and then secure the prey using constriction. Next they maneuver their mouths forward ensuring that the last maxillary tooth punctures the skin allowing the venom to enter the prey’s tissue. Ring-necked snakes are rarely aggressive to larger predators suggesting that their venom evolved as a feeding strategy rather than a defense strategy. Rather than trying to bite a predator, the snake winds up its tail into a corkscrew, exposing the brightly colored belly.
Image sources: , , 
The 3 main types of hip joint in tetrapods.
Typical of reptiles, dinosaurs & mammals, and rauisucians respectively.
older vs modern reconstructions
Helicoprion (“Spiral Saw”) was a long-lived genus of shark-like cartilaginous fish that first arose in the oceans of the late Carboniferous 280 million years ago, survived the Permian-Triassic Extinction Event, and eventually went extinct during the early Triassic, 225 million years ago.
The only fossils known are the teeth, which were arranged in a fantastic “tooth-whorl” strongly reminiscent of a circular saw. It was not until the discovery of the skull of a relative, Ornithoprion, that it was realized that the tooth-whorl was in the lower jaw. The tooth-whorl represented all of the teeth produced by that individual in the lower jaw, in that as the individual grew, with the older, smaller teeth being moved into the center of the whorl by the appearance of larger, newer teeth. Comparisons with other eugenodontids suggest that Helicoprion may have grown up to 10-15 ft (3-4 m) long.
The exact location of the tooth-whorl in the lower jaw is an open debate. Older reconstructions placed the whorl in the front of the lower jaw; however this would create drag, making the shark a less efficient swimmer, and turbulence, alerting prey of its approach. A more current and scientifically accepted reconstruction places the whorl deeper into the throat. This arrangement would be best suited for soft bodied prey.
Coloured modern reconstruction illustration  by Mary Parrish
Other images currently unsourced
The Armadillo Lizard (Cordylus cataphractus) is a lizard endemic to desert areas of southern Africa. It is also known as the Typical Girdled Lizard, Armadillo Girdled Lizard, Golden Armadillo Lizard, and Armadillo Spiny-tailed Lizard.
The Armadillo Lizard possesses an uncommon antipredator adaptation, in which it takes its tail in its mouth and rolls into a ball when frightened. In this shape it is protected from predators by the thick, squarish scales along its back and the spines on its tail. This behavior, which resembles that of the mammalian armadillo, gives it its English common name. This behavior may have inspired tales of the mythical creature Ouroboros.
Galerie de paléontologie et d’anatomie comparée, Paris
Strong contender for my favourite species.
The fossa [pronounced /”FOO-sa”/ in Malagasy] (Cryptoprocta ferox) is a cat-like, carnivorous mammal that is endemic to Madagascar. It is a member of the Eupleridae, a family of carnivorans closely related to the mongoose family (Herpestidae). Its classification has been controversial because its physical traits resemble those of cats, yet other traits suggest a close relationship with viverrids (most civets and their relatives).
The fossa is the largest mammalian carnivore on the island of Madagascar. Several of the animal’s physical features are adaptions to climbing through trees. It uses its tail to aid in balance and has semi-retractable claws that it uses to climb trees in its search for prey. It has semiplantigrade feet, switching between a plantigrade-like gait (when arboreal) and a digitigrade-like one (when terrestrial). The soles of its paws are nearly bare and covered with strong pads. The fossa has very flexible ankles that allow it to readily grasp tree trunks so as to climb up or down trees head first or to leap to another tree. Captive juveniles have been known to swing upside down by their hindfeet from knotted ropes.
The fossa is listed as “Vulnerable” by the International Union for Conservation of Nature (IUCN). It is generally feared by the Malagasy people and is often protected by their taboo, known as fady. The greatest threat to the species is habitat destruction
So I do a Google image search for “amphibian skull”
And these are the first two images that show up: