The Clymeniida constitute a unique order of ammonoid cephalods from the Upper Devonian characterized by having an unusual dorsal siphuncle. They measured about 4 cm in diameter and are restricted to Europe, North Africa, and possibly Australia.

 Clymeniids produced a variety of shells ranging from smooth to ribbed and spinose and from evolute with all whorls exposed to strongly involute with the last whorl covering the previous. Some were even triangular as viewed from the side (along the axis of coiling). With the exception of the first few chambers, all have a siphuncle that runs along the dorsal margin, along the inside of the coils, opposite that of most ammonoids.

 The siphuncle starts off ventrally, like that in other ammonoids, but after the first few septa migrates to a definite and stable dorsal position. Septal necks are retrosiphonate, characteristic of their nautlloid ancestors, and are commonly very long, forming an almost continuous siphunclular tube. Septa, characteristic of ammonoids, are convex toward the front.

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Tags: Аммониты, Вымершие беспозвоночные, Девон

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