258 



STUDIES IN EVOLUTION 



equal lobes of the mantle, v being the ventral lobe, and d 

 the dorsal; h is the head, and jt? the caudal segment develop- 

 ing into a pedicle. A deposit of integument representing 



101 



103 



106 



Thecidium. (Lacazella) mediterraneum Risso. 



Figure 101. — Cephalula; dorsal side, ds, dorsal shell plate; h, head. 

 (After Kovalevski.) 



FiGLRE 102. — Dorso-ventral longitudinal section of cephalula of about same 

 age as preceding. A, head ; d, dorsal mantle lobe ; v, ventral mantle lobe ; 

 ds, beginning of dorsal valve ; del, shell plate forming on dorsal side of body ; 

 p, pedicle. (After Kovalevski.) 



Figure 103. — Typembryo ; larva transformed from folding upward of 

 mantle lobes, h, head ; ds, dorsal valve ; hi, hinge-line of dorsal valve ; del, 

 shell plate on body and pedicle posterior to hinge-line of dorsal valve. (After 

 Kovalevski.) 



Figure 104. — Dorso-ventral longitudinal section of preceding. References 

 as in figure 103. vs, ventral valve; ]>, pedicle. 



Figure 105. — I'rofile view of neanic Lqitana rhomboidalis. The features of 

 the shell are placed and lettered as in figure 104. ds, dorsal valve; hi, hinge- 

 line ; del, deltidium ; p, pedicle-opening ; vs, ventral valve. 



Figure 106. — Adult Thpcidium {Lacazella) mediterraneum; dorsal side; 

 showing ventral area and deltidium. 



Figure 107. — Profile of same. References as in figures 104 and 105. 



the shell has formed on the inner side of the dorsal mantle 

 lobe Qls), and also on the adjacent dorsal side of the body 

 lobe (jleT). A larva somewhat more advanced is represented 

 in figure 101, as viewed from the dorsal side. The mantle 



