574 Forty-fifth Report on the State Museum. 



Trematis, Sharpe. 1847. 

 ' (Plate 6, Figs. 1-8.) 



Shell subcircular or transversely oval in outline. Pedicle-valve 

 unevenly convex, more or less depressed over the posterior region; 

 apex at, or behind the center; directly beneath it begins the 

 pedicle-fissure, which transects the shell, vertically widening to 

 posterior margin with straight or outwardly curving edges. 

 Brachial valve evenly convex, with its apex marginal and slightly 

 projecting. On the interior, the pedicle-valve shows a faint 

 median furrow extending' from the angle of the fissure to the 

 apex of the shell ; this groove widens at its apical termination 

 and may represent a point of muscular attachment. The sides 

 of the fissure are often thickened by callosities similar to those 

 sometimes seen in species of Orbicdloidea. From the apex of 

 the valve extend radiating and branching vascular sinuses. 



In the brachial valve the posterior margin is much thickened 

 and broadly grooved to allow the extrusion of the pedicle. This 

 thickening does not take the form of a cardinal area or shelf, 

 but is rather a callosity closely appressed against the interior 

 surface of the shell, the central portion being projected beyond 

 the margin of the pedicle-valve. Directly below and in front of 

 this area are two transversely elongate scars, adjustors or poste- 

 terior adductors, which are usually partly concealed by the pro- 

 gressive overgrowth of the cardinal thickening. A faint median 

 septum begins between these scars and passes forward, becoming 

 more prominent over the tongue-shaped median elevation which 

 separates the large central scars. These impressions are oblique 

 and are not simple, each appearing to be composed of two, if not 

 three distinct scars, making a posterior, a median and an anterior 

 pair. What appears to be the posterior pair is small, and some- 

 times quite sharply defined, the central pair very much large;', and 

 the anterior pair narrow, situated at either side of the angle of the 

 median callosity and separated by its apex The specialization 

 of the first of these scars is not satisfactorily established ; the 

 entire impression is deeply excavated. In some well preserved 

 specimens, there is also evidence of external, marginal scars lying- 

 just in front of the outer ends of the posterior adductors. 



Surface; of both valves more or less completely covered by a 

 beautiful ornamentation consisting of punctures or small pittings 



120 



