316 RHYNCHONELLTD^. 



Shell like that of Rhynchonella ; the lower half of the ventral 

 valve with a broad, deep sinus. Yalves articulating by means of 

 two teeth in the ventral valve, with corresponding sockets in the 

 dorsal valve, and a median septum embraced between the deeply'' 

 bifurcating cardinal process of the opposite one. 



Dorsal valve with four crural processes ; in the ventral valve 

 the dental plates are represented by elevated lamellae surrounding 

 the muscular impression, which is much stronger and differs in 

 some respects from that of Rhynchonella, 



DlMERELLA, Zittcl, 1870. 



Syn. — Cr^qDtopora, Jeffreys, 1869. Atretia, Jeffreys, 1816. 



Diatr. — Recent. D. gnomon^ Jeffreys, Europe. Fossil; 

 Triassic. 



Shell small, Rhynchonelloid, impunctate, with a large fora- 

 men; neural valve with an entire edge without a septum ; haemal 

 valve with a large, very prominent septum, which divides the 

 cavity of the shell, when closed, into two chambers; with two 

 stout, diA^erging hook-shaped crura as in Rhynchonella, 



Rhynchonellina, Gemellaro, 1811. 



Bifftr. — 4 sp. Jurassic ; Sicily. B. Suessi, Gemm. 



Shell quadrangular or triangular, smooth or radiaUy ribbed, 

 hinge-margin nearly straight ; large valve swollen, with curved 

 beak, a large triangular area, with a rudimentary deltidium and 

 oval opening ; teeth and pits as in Rhynchonella ; upper valve 

 less swollen or flatfish, with two verj^ long crura, almost reaching 

 the opposite valve, these crura sometimes provided, near the 

 hinge, with sickle-shaped processes. 



Stricklandinia, Billings, 1863. 



Etym. — Dedicated to the late Professor H. E. Strickland. 



Syn. — Stricklandia, Billings, 1859 (non Buckman). Rens- 

 selaeria (pars). Hall, 1859. 



Biatr. — 10 sp. Silurian ; N. America, England. S. elongata, 

 Vanuxem, is the only species known in the Devonian rocks. S. 

 Ze?is, Billings (cxxxvi, 80, 81), 



Shell usually large, elongate-oval, etc. ; valves nearly equal, ■^ 

 never globose ; a short mesial septum in the interior of the 

 ventral valve supporting a small triangular chamber beneath 

 the beak as in Pentamerus ; in the dorsal valve no longitudinal 

 septa, spires, or loop, the whole of the internal solid organs 

 consisting of two short or rudimentary dental plates, which in 

 some species bear prolonged calcified processes for the support 

 of the cirrated arms. A more or less developed area in the 

 ventral valve. 



In *S'. Jserdi^ and S. m.icrocameriis the hinge-line is straight and 

 much extended. In S. A7'achne, Billings, the area of the ventral 



