211 



muscles of the dorsal valve. In Obolus, sens, strict., these muscles are 

 well apart. 



The following is the original description of this species : 



Shell oblately "orbicular, lenticular, edges thin, sharp, flat. Test corneous, 

 surface brilliant. 



The ventral valve is produced at the back into a low, rather blunt beak, Description of 

 depressed at the point. In the interior of the ventral valve, there are at valves 

 the back three ridges, which diverge from the umbo ; of these the mesian 

 ridge dividing the posterior adductor muscles, is short and weak ; it is 

 forked at the end (where it terminates against the scar of the pedicle 

 muscle ?) The two lateral ridges are longer, being about one-quarter of 

 the length of the shell, and are more distinctly outlined, by a long narrow 

 pit on the inner side. The visceral cavity is wide in front and terminates 

 in a long sinus ; some specimens show in this sinus a muscular callosity. 

 The hinge area is narrow, and deflected downward in the middle. It is 

 crossed by a shallow pedicle groove. 



The dorsal valve has at the hinge a very thin edge, turned inward. The 

 interior is marked by three diverging ridges, of which the central is longer 

 than the lateral ones. The central ridge when well preserved, appears to 

 extend two-thirds of the length of the shell, and is distinct as far as the 

 middle of the shell, where there are two small muscular impressions ; at 

 the back of the shell, between the mesian and lateral ridges, are the 

 impressions of the [posterior] adductor muscles. There is a large shallow 

 bilobed depression in the anterior half of the shell. The cardinal border 

 is thin and is made much more distinct by a pair of small narrow trans- 

 verse pits, one on each side of the mesian ridge. 



Sculpture (1). The surface is marked by very fine concentric and radiat- 

 ing lines, and there are stronger concentric growth lines at intervals on 

 the surface. 



Size. Length of the ventral valve 8^ mm. ; width 10 mm. The dorsal 

 valve is half a milimetre shorter than the ventral. 



Horizon and locality. In the Dictyonema shales (Div. C. 3c) Navy 

 island, St. John harbour. 



No very satisfactory examples of the marking of the interior of the 

 valves were found, in any one individual ; but by combining the markings 

 found on several valves of the dorsal and ventral sides respectively, it has 

 been found possible to restore some of the features of the interior of the 

 shell." 



H| c. E. 



