214 SYNOPTIC COLLECTION. 



PL 431. This stage succeeds the trochophore, and is 

 known as the veliger a characteristic stage in the devel- 

 opment of Gastropods. The ciliated velum still exists 

 and the foot is also formed. 



In most Gastropods this cone-like shell becomes a 

 spiral, but in Chitons (No. 432, C. magnificus Desh.), 

 which may be primitive Gastropods, 1 the one-valved shell 

 is made of eight pieces. In No. 433 these pieces are 

 separated and are seen to be essentially alike. 



Chiton existed in the Silurian age and has undergone 

 comparatively little change since that time. The animal 

 agrees with many Pelecypods in being bilaterally symmet- 

 rical. On the other hand, the head is indistinctly 

 marked off from the rest of the body and there is a well 

 developed lingual ribbon (an apparatus for eating) which 

 is possessed by Gastropods. 



The cone-like shell may be represented by Tryblidium 

 (PI. 434, T. nycteis Billings) from the Cambrian, which 

 had a smooth cap-like shell when young with an entire 

 margin, becoming in the adult like a shallow cone (PL 

 434, fig. i) with the apex placed near the forward end 



(fig. 2). . 



As we have just said, the cone of most young Gastro- 

 pods becomes a spiral and this youngest spiral shell or 

 protoconch (PL 43 5 ) 2 is smooth, rounded, and light col- 

 ored. It is formed at the apex of the shell but is usu- 

 ally broken off and lost. 



In the Ordovician fauna, Pleurotomaria (No. 436, P. 



1 Conflicting views are held in regard to the position of the Chi- 

 tons. Some naturalists place them before the Pelecypods, while 

 others assign them a position between the Pelecypods and Gastro- 

 pods. They are here placed provisionally among the more primi- 

 tive Gastropods. 



2 Although this is the protoconch of Fulgur, (see p. 223 and Nos. 

 462-465), a more specialized species than those we are now describ- 

 ing, yet it is placed here for the purpose of showing the general 

 characters of the Gastropod protoconch. 



