84 CRUSTACEA CIRRIPEDIA 



Sub-Order 1. Pedunculata. 



In this division, sometimes combined with the Operculata 

 as THORACICA, owing to the extremely reduced state of the 

 abdomen, the body is borne on a distinct stalk, and the bivalve 

 arrangement of the mantle is clearly retained. The mantle is 

 protected externally by a number of calcareous plates, the 

 arrangement of which is typical of the various genera. It 

 appears that in the most primitive and geologically- oldest 

 Cirripedes, the probable ancestors of the Pedunculate and Oper- 

 culate sub-orders, the arrangement of the plates was somewhat 

 irregular, and they were far more numerous than in the modern 

 forms, so that passing from these older types to modern times 

 we witness a reduction in the number and a greater precision 

 in the arrangement of the skeletal parts. 



One of the most ancient Cirripedes known is Turrilepas, which 

 occurs in the Silurian deposits of England, but it is also known 

 from earlier deposits, while undoubted 

 /~f Cirripedes have been found in the Cam- 

 brian of North America. The body of 

 Turrilepas is enclosed in imbricating 

 plates, as shown in Fig. 53, A. 



In Archaeolepas of the Upper Jurassic 

 (Lithographic slates of Bavaria) the ar- 

 rangement of scutes typical of the Lepa- 

 didae is foreshadowed, but the whole 

 of the peduncle is protected by rows 

 l; B, Archaeolepas redten- of plates (Fig. 53, B), as in Turrilepas. 



backeri, (Jurassic), x 1. C. mi i T T 



carina ; R, rostrum; s, -L ne above-mentioned genera did not 



scutum ; T, tergum. (After survive into the Cretaceous period, their 



places being taken by the genera Pollicipes 



and Scalpellum, which first appeared in the Silurian and persist to 

 the present time, the older and more primitive Poll-it- ipf* being 

 represented by about half a dozen living species, while the species 

 of Scalpellum are exceedingly numerous. 



Fam. 1. Polyaspidae. This family includes the three genera, 

 Pollicipes, Scalpellum, and Lithotrya. 



Pollicipes is not only very ancient geologically (being found from 

 the Ordovician upward), but it preserves the primitive character- 



