204 



MOLLUSCA. 



in the Devonian period, the latter in the Oolitic, and both 

 being represented by living forms. In the genus Waldheimia 

 there is a very long loop, which is bent backwards, and the 

 same is the case with Terebratella. The former appears to 

 commence in the Trias, the latter in the Cretaceous Rocks, and 

 both have survived to the present day. Forming a section of 

 the Terebratulidce, or sometimes regarded 

 as a separate family, are the two or three 

 species which make up the genus Strin- 

 gocephalus. These (fig. 144) are all De- 

 vonian, and are characterised by the pos- 

 session of a long loop, and a widely- 

 punctated shell. The beak of the ventral 

 valve is very prominent, and is pierced by 

 a foramen, which is large in the young, 

 and small in the adult shell ; and the ventral valve has a well- 

 developed mesial septum. 



FAM. II. THECIDID^E. Shell fixed to the sea-bottom by 

 the substance of the beak of the larger or ventral valve ; 

 structure punctated. Oral processes (arms) united in the form 



Fig. 144. Stringoce- 

 phalns Burtini, Devon- 

 ian. 



T- 



..r 



Fig. 145. Thecidium papillatnm. e Hinge-area ; n Hinge-teeth of ventral valve ; 

 r r Granulated border of the interior of the dorsal valve. 



of a bridge over the visceral cavity ; cirrated arms folded upon 

 themselves, and supported by a calcareous loop. (Fig. 145-) 



