fhe Cirripede Oemia Sframentiim. 79 



probably be found to be quite as interesting^ as StramenUim. 

 lu addition to the valves known in the capitulum of Stra- 

 inentum, Squama is said to possess a rostrum, subrostrum, 

 and subcarina, but whether tliese latter valves are keeled 

 or whether tliey have the same structure as the carina in 

 Stramentmn is not known. 



Stramentum is known in the Cenoraanian only by one 

 specinjen from Syria, but is comparatively common in the 

 Turonian of Europe. It lingers on to the Senonian in 

 Europe, but is exceedingly rare, while in the Senonian 

 Kansas Chalk of America it is common. We have, there- 

 fore, to look in the earlier Cretaceous and Upper Jurassic 

 rocks for the ancestors of Stramentum. By some authors 

 Stramentum is considered to be a derivative from the Ju- 

 rassic genus Archceolepas, but it must not be overlooked that 

 that genus is not entirely confined to the Jurassic, since one 

 or two species range into the Lower Cretaceous. 



Archceotepas has a capitulum of six plates only, consisting 

 of paired scuta, paired terga, and a carina and rostrum of 

 the type seen in the Scalpellidse (Sci/lcelepas), although 

 the carina is much reduced in size. The earliest form, 

 namely, the genotype Archceolepas redtenbacheri, is known 

 to me only by figures, which would appear to be unreliable, 

 since they differ in the number of vertical rows of pedun- 

 cular plates. Zittel's figure is probably more correct 

 and shows five rows, and the disposition of the plates 

 resembles that of Strameritum more than any other form 

 of cirripede. There is a close resemblance in the shell 

 of Archccohpas {A. redtenbacher'i) to that of Stramentum, 

 and a further point in common is the reduced carina. The 

 general structure of the shell of that early form of Arcliceo- 

 lepas also points to the probability that, as in Stramentum, 

 the greater part of the animal's body was lodged in the 

 peduncle. In this connexion it is interesting that a much 

 more definite peduncle is developed in the Portlaiuliau 

 species PolUcipes royeri, which is undoubtedly an Archceo- 

 tepas ; and in the Cretaceous (Ncocomian) Archceolcpas 

 decora, the peduncle is well defined and almost twice as 

 long as the capitulum. There ap[)ears, therefore, to be 

 some relation in ArcJueolepaa between the geological age of 

 the form and the degree of differentiation of tlie peduncle 

 from the capitulum. 



It might well be that Archaolepas and Stramentum were 

 originally derived from the same stock, but the two forms 

 were certainly well differentiated in the Cretaceous. Archao- 

 lepas evidently developed into a fcji-m with a well-dt'fincd 

 peduncle, and there is no douljt that it rc[)rcs('n(s one of the 



