OF CONCHOLOGY. 



17 



pretty regular and strong imbricating concentric ridges, and by 

 radiating plications ; the latter most marked on and to^vards the 

 edges of the imbrications. Lower valve variable, flat, slightly 

 convex or slightly concave. In two instances out of five speci- 

 mens this valve appears to have been attached to another body 

 during the life of the animal ; and in one of these cases, where 

 it is now separated, while the chai*acteristic markings are discern- 

 ible, they are blurred, and combined with oblique striation from 

 the other surface. In at least one other case this valve seems 

 to have been entirely free, and the radiating sculpture is even 

 stronger than the concentric. 



Dimensions of the largest specimen : length from beak to base, 

 1-3 in.; width, 1-05 in.; height of deep valve, -2 in. From a 

 smaller one : length, as above, 1 in. ; width, '95 in. ; diameter of 

 both valves, '25 in. 



Locality. Brought from the Jurassic of Volcano, by both 

 Mr. Clayton and Dr. Blatchley. One specimen is in the same 

 block with the Astarte, above described. 



This shell resembles P. spinosa, Sby., of the European Lias, 

 but is less oblique and has many more of the concentric imbri- 

 cating ribs. 



SPIRIFER, Sby. 



S. OBTUsus, Gabb. — PI. 7, fig. 16, a, b. 



Shell robust, short, thick, valves very convex ; base regularly 

 convex, nearly semi-circular, lateral angles rounded ofi:' and con- 

 verging into the hinge line ; dorsal valve almost as long as wide, 

 deep ; beak elevated, somewhat incurved ; mesial sinus regularly 

 concave, strongly marked ; area shorter than the width of the 

 shell, very broad, foramen large ; ventral valve less convex 

 than the dorsal ; beak small, incurved ; mesial fold elevated, 

 sub-angulated and bearing three ribs on the upper surface and 

 one on each side on the slope. Surface ornamented by about 

 twenty-three or twenty four rounded ribs on each valve, of 

 which three or four belong to the mesial fold and sinus. These 

 ribs are separated by equal, concave interspaces ; all crossed by 

 small, sub-imbricating lines of growth. 



Length from apex to base of ventral valve, 1*3 in. ; of dorsal 

 valve, 1 in. ; width, 1-35 in. ; length of area, 1-15 in. ; distance 

 between the two beaks, '28 in. ; greatest diameter of two valves, 

 •95 in. 



Locality. Two specimens from the Trias of Volcano, from 

 Dr. Blatchley. 



