﻿T. Davidson — What is a Brachiopod ? 207 



greater or lesser length, wliioli coalesce into one median plate, and 

 then diverge to form the dental plates, inclosing a triangular trough- 

 like chamber. In the interior of the dorsal valve are two longitudi- 

 nal septa of variable dimensions, to which the socket- walls converge, 

 and to which they are joined, forming two more or less developed 

 and inclined plates, to the produced extremities of which were no 

 doubt affixed the fleshy spiral labial appendages. Shell structure 

 irapunctate. Genera : Pentamerus, Pentamerella, and perhaps one 

 or two others. 



Family 10. Strophomenida. — Shells semicircular, transverse, or 

 elongated : valves usually concavo-convex, regularly arched, geni- 

 culated or depressed, so that the valve, which is convex in some 

 species, is concave in others, and vice versa; hinge-line long, straight: 

 area in ventral valve flat, with a fissure partly arched over by a 

 pseudo-deltidium. Yalves sometimes uniformly convex, the dorsal 

 one sometimes depressed with an area divided by a triangular 

 foramen. In the interior of the dorsal valve a small simple pro- 

 jecting cardinal process is situated between prominent socket- walls, 

 to the inner extremities of which were (probably) attached the brachial 

 appendages. Genera : Sti-ophomena Streptorliynclius, Stropliodonta, 

 Leptmia, Orthis, Orthesina, Skenidium, Brachy2mon, Disccelosia, Meehella, 

 Davidsonia (?), and several others. The first species appeared during 

 the Silurian period, and the last in the Upper Lias. It may, how- 

 ever, be necessary to group the genera provisionally placed in 

 StrophomenidiB into one or two families or sub-families. A great 

 family Orthidcs might be established. Stropliomena differs from 

 OrtMs in having a closed fissure, and the cardinal process bifid or 

 trilobed, while in OrtMs it is generally formed of one piece. In 

 StropJiomena it is situated directly between the dental sockets, or has 

 between them and it a small prominent ridge, or brachial process ; 

 for this last is scarcely developed, where it exists, and forms a marked 

 contrast to what we find in same valve of Orthis. There are also 

 four more or less distinctly defined adductor depressions, which are 

 longitudinally parallel to each other, and separated by ridges, while 

 in Orthis these four divisions are placed in pairs one above the other. 



Family ] 1. Productidce. — Shells more or less concavo-convex, 

 oval, semi-oval, or angular, and generally auriculated ; the hinge- 

 line straight, with or without teeth and sockets for the articulation 

 of the valves. Surface of ventral valve or hinge-line more or less 

 furnished with tubular spines, sometimes of considerable length : no 

 calcareous process for the support of the brachial appendages : shell 

 structure perforated by canals : cardinal process prominent, trilobed 

 or bilobed. Under this a narrow longitudinal ridge generally ex- 

 tends to about half (or more) of the length of the valve, and on each 

 side are seen the ramified dendritic impressions, which may be attri- 

 butable to the adductor muscle. Outside, and in front of these are 

 the two reniform impressions so characteristic of the family. Genera : 

 Productus, Strophalosia, Aulosteges, Chonetes, Productella. The Pro- 

 ductidce made their first appearance during the Silurian time, and 

 became extinct at the close of the Paleeozoic period. 

 (To be concluded in our next 2fumber.) 



