THE CRUSTACEA 



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

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

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

 PoUicipes, Avhich dates back to the Silurian, possibly to the 

 Ordovician epoch. In Pollkipes the peduncle is covered with small 

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

 and in some cases (P. mius (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, Avith a varying number of lateral plates, forming a 

 transition to the scales of the peduncle. 



From the type of Pollkipes two lines of modification may be 



V .^ ^ ,^^\^V 



Fig. 60. 



■ PoUic'rpes sertus, showing 

 tran.sition from peduncular 

 spales to capitular plates. 

 (From Gruvel's Monographic.) 



Fio. 61. 



Scalpdhimstcarnsii, x §. 

 (After Stebbing, from En- 

 cijcl. 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 

 Dichelaspis, to Alepas and Anelasnm, where the mantle is entirely 

 membranous. 



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

 originated from a form resembling PoUicipes, or intermediate 

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

 Avhorl 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 opei'culum. The most primitive genus among the 



