T. Davidson — What is a Brachiopod ? 
207 
gi'eater or lesser length, which coalesce into one median plate, and 
then diverge to form the dental plates, inclosing a triangulär 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 
impunctate. Genera : Pentamerus, Pentamer ella, and perhaps one 
or two others. 
Family 10. Strophomenidce. — 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 triangulär 
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 : Strophomena Streptorhynchus, Strophodonta, 
Leptcena, Ortkis, Orthesina, SJcenidtum, Brachy prion, Disccelosia , Meeicella, 
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 
Strophomenidce into one or two families or sub-families. A great 
family Orthidee might be established. Strophomena differs from 
Orthis in having a closed fissure, and the Cardinal process bifid or 
trilobed, while in Orthis it is generally formed of one piece. In 
Strophomena it is situated directly between the dental sockets, or bas 
between them and it a small prominent ridge, or brachial process ; 
for this last is scarcely developed, wliere 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 11. 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 tubulär 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 : 
Prodnctus, Strophalosia, Aulosteges, Chonetes, Productella. The Pro- 
ductidce made their first appearance during the Silurian time, and 
becarne extinct at the close of the Palasozoic period. 
(To be concluded in our next Number.) 
