REPTILIAN AGE. JURASSIC PERIOD. 71 



in size and form, and placed under the beak, by the incurving of which it is often 

 closed or hidden; partly or entirely surrounded (the inner side being sometimes 

 formed by the umbo of the smaller valve) by a deltidium, which is composed of two 

 pieces, and merely rudimentary, or more or less weU developed — sometimes produced 

 in the form of a short tube. Hinge composed of two teeth in the larger valve, and 

 two corresponding sockets in the other ; the teeth being supported by dental plates 

 which extend to the bottom of the valve. Apophyses of the smaller valve con- 

 sisting of two short, flattened, moderately curved, and separate laminae, which 

 curve upwards and are attached to the hinge plate. Impressions of the adductor 

 muscle in the smaller valve, quadruple, well defined, and separated by a short 

 longitudinal mesial ridge; scars of the pedicle muscles occupying the cardinal 

 plates. Shell and pedicle muscles of the larger or perforate valve occupying a 

 saucer-shaped cavity at the base of the dental plates ; those of the pedicle muscles 

 narrow, elongate, and placed close to the inner bases of the dental laminae — the 

 remaining and larger portion of the cavity being chiefly occupied by the cardinal 

 muscles, which are separated by a small, ridge ; above these is the small oval 

 adductor scar. 



Animal of R. psittacea, according to Mr. DaAidson, with visceral mass confined 

 to a small space near the beaks, and separated from the general cavity of the shell 

 by a strong membrane, in the middle of which tlie mouth is situated. Upper lip 

 plain, and the lower cirrhated. Alimentary canal passing through the deeply notched 

 hinge plates, and terminating behind the point of insertion of the adductor muscle 

 in the centre of the valve. Pallial veins consisting of four principal branches in 

 each lobe, opening into larger sinuses. Margins of mantle fringed with a few 

 short setae. 



This genus is of very ancient date, having been represented through the various 

 geological epochs from the Silurian down to the present time. The species were 

 quite abundant during some of the Palaeozoic periods, as well as during the deposi- 

 tion of some of the Jurassic and Cretaceous rocks, particiilarly in Europe. The 

 genus seems to be sparingly represented in the Tertiary deposits, and at present 

 but two or three living species are known. The recent species are never so strongly 

 costated or plicated, nor so short and subtrigonal in form as many of the older 

 extinct ones, which, together with some other difi'erences, have led several authors 

 to think them not strictly congeneric. 



Rhynchonella . 



(Plate III, Fig. 4.) 



Our specimens of this shell are too imperfect to enable us to determine satisfac- 

 torily whether or not it is identical with any known form. In a genus like this, 

 including so many species, often very closely allied, not much reliance can be placed 

 upon identifications from a few imperfect specimens. It would be easy to point out 



