the Cirripede Genus Stramentum. 79 
probably be found to be quite as interesting as Stramentum. 
In addition to the valves known in the capitulum of Stra- 
mentum, Squama is said to possess a rostrum, subrostrum, 
and subcarina, but whether these latter valves are keeled 
or whether they have the same structure as the carina in 
Stramentum is not known. 
Stramentum is known in the Cenomanian only by one 
specimen from Syria, but is comparatively common in the 
Turonian of Europe. It Jingers 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 Archeéolepas, 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. 
Archeolepas has a capitulum of six plates only, consisting 
of paired scuta, paired terga, and a carina and rostrum of 
the type seen in the Scalpellide (Scillelepas), although 
the carina is much reduced in size. The earliest form, 
namely, the genotype Archeolepas 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 Stramentum more than any other form 
of cirripede. There is a close resemblance in the shell 
of Archeolepas (A. redtenbacheri) 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 Archeo- 
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 Portlandian 
species Pollicipes royeri, which is undoubtedly an Archeo- 
lepas; and in the Cretaceous (Neocomian) Archeolepas 
decora, the peduncle is well defined and almost twice as 
long as the capitulum. There appears, therefore, to be 
some relation in drcheolepas between the geological age of 
the form and the degree of differentiation of the peduncle 
from the capitulum. 
It might well be that Archeolepas and Stramentum were 
originally derived from the same stock, but the two forms 
were certainly well differentiated in the Cretaceous. _Archeo- 
lepas evidently developed into a form with a well-defined 
peduncle, and there is no doubt that it represents one of the 
