110 



THE CRUSTACEA 



plates of the terminal row on each side are much enlarged and 

 represent the capitular plates. Much older than Loricula, however, 

 and probably much more primitive, is the still-existing genus 

 Pollicipes, which dates back to the Silurian, possibly to the 

 Ordovician epoch. In Pollicipes the peduncle is covered with small 

 scale-like plates which increase in size towards the capitular end, 

 and in some cases (P. sertus (Fig. 60)) show a complete gradation 

 of size and arrangement up to the capitular plates themselves. 

 The latter consist of unpaired rostrum and carina, with paired terga 

 and scuta, with a varying number of lateral plates, forming a 

 transition to the scales of the peduncle. 



From the type of Pollicipes two lines of modification may be 



FIG. 60. 



Pollicipes serins, showing 

 transition from peduncular 

 scales to capitular plates. 

 (From Gravel's Monographic.) 



FIG. 61. 



Scalpel! timstcvDixii, x ?,. 

 (After Btebbing, from A- 

 cycl. Brit.) 



traced. On the one hand, in the group Pedunculata, we find the 

 scales of the peduncle becoming horny and disappearing, while 

 the capitular plates diminish in number as we pass from 

 Scalpellum (Fig. 61), through forms like Lepas (Fig. 57, A) and 

 Dicfielaspis, to Alepas and Anclasma, where the mantle is entirely 

 membranous. 



On the other hand, the Operculata may be supposed to have 

 originated from a form resembling Pollicipes, or intermediate 

 between it and Loricula. The peduncle has disappeared, but the 

 whorl of plates immediately below the capitular valves have 

 persisted, and, together with the rostrum and carina, have become 

 united to form the outer " wall " of tubular or conical form, within 

 the opening of which the scuta and terga are articulated to form 

 the movable operculum. The most primitive genus among the 



