BRACHIOPODA. 443 



the present day. Terebrirostra has a greatly elongated beak 

 to the ventral valve, and appears to be confined to the Creta- 

 ceous period. Another Cretaceous form is the Trigonosemus 

 of Konig, which is closely allied to Terebmtella, especially in 

 the fact that the brachial loop has a secondary attachment 

 to a mesial septum, but which has an incurved beak, a large 

 cardinal process, and an extensive hinge-area. Rensselceria, 

 again, of the Devonian, is essentially a large ovoid Terebmtula. 

 The genus Argiope, ranging from the Cretaceous to the pres- 

 ent day, is distinguished from Terebratula by the form of 

 its brachial loop, which is attached to a single septum, or to 

 three nearly parallel septa, directed inwards from near the 

 border of the dorsal valve. In the Cretaceous Magas an 

 even more highly developed septum is present, almost divid- 

 ing the cavity of shell into two halves ; while in the Megerlia 

 of the same formation the brachial loop has an exceptionally 

 complex character. 



Lastly, we may place in the Terebmtulidce the Devonian 

 genus Centronella, and the seemingly related Leptocodia ot 

 the Upper Silurian and Devonian. Forming a section of the 

 Terebratulidce, or sometimes regarded as a separate family, 

 are the two or three species which make 

 up the genus Stringocephalus. These (fig. 

 282) are all Devonian, and are charac- 

 terised by the possession 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 



-, .-, , ,, . ,, Fig. 282. Stringoce- 



large in the young, and small in the p ha ius 



adult shell; and the ventral valve has a ian - reduced in size - 

 well-developed mesial septum. 



FAM. II. THECIDIIM. 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 

 of a bridge over the visceral cavity; cirrated arms folded 

 upon themselves, and supported by a calcareous loop. The 

 shell is thickened, and has a granulated border (fig. 283). 



The members of this family are all attached to some foreign 

 body by a portion of the beak of the ventral valve, which, 



