BRACHIOPODA. 



6 75 



a single triangular crural process" (Billings). The species of this genus 



seem to be mainly confined to the Palaeozoic rocks of North America ; 



but European forms are 



known. The Silurian and c a b 



Devonian genus Leptoca'lia 



seems to be nearly allied 



to Centronella,) but the shell 



is radiately-ribbed, whereas in 



the latter genus it is smooth. 



The Silurian and Devonian 



genera Retisselceria and Me- 



gantcris likewise include 



ancient types of the Terebra- 



tulidcB. 



In the genus Terebratella 

 (fig. 545) the shell is some- 

 times smooth, sometimes ra- 

 diately-ribbed, and there is 



an incomplete deltidium (m) below the foramen. In the dorsal valve 

 is a well-developed mesial septum, with which the brachial loop becomes 

 secondarily connected by means of transverse calcareous processes, one 

 on each side. The genus Terebratella begins in the Lias, is well repre- 

 sented in the Cretaceous formation, and still exists at the present day. 



Various Mesozoic, Tertiary, and Recent Brachiopods are closely re- 



Fig. 544. — Lateral (a) and dorsal (b) views of Cen- 

 tronella glans-fagea, from the Devonian rocks of 

 North America, of the natural size ; c, Brachial loop 

 of the same, enlarged. (After Hall — copied from 

 Zittel.) 



Fig. 545. — Terebratella Astieriana — Cretaceous, e, Hinge-area; ?;z, Deltidium. 



lated to Terebratella, with which they agree in the fact that the brachial 

 loop, in addition to its normal attachment to the hinge-line, has a second- 

 ary connection with a median septum in the dorsal valve. Among the 

 forms in question, Lyra ( Terebrirostra) has a greatly elongated beak to 

 the ventral valve, and is confined to the Cretaceous period. The genus 

 Trigonosemns, also Cretaceous, has an incurved beak, a large cardinal 

 process, and an extensive hinge-area. In the genus Afagas, of the Cre- 

 taceous period, the dorsal septum is so greatly developed as almost to 

 divide the cavity of the shell into two halves ; while in Megerlia (Chalk 

 to Recent) the brachial loop is not only attached to the hinge-plate, but 

 is doubly connected with the dorsal septum. The Jurassic and Creta- 

 ceous Kingena, and the recent Boucliardia and Kraussina are other 

 types belonging to the same group. 



Family 8. Argiopid/E. — In this family there is a large foramen 

 for the transmission of the peduncle of attachment, and the dorsal 



