347 



impressions of the ribbing of Pecten acutiradiatus were apparent. 

 The hinge of the left valve presents, under the umbo, the same 

 elongated triangle whose apex extends to the external contour, 

 and is formed by two salient ridges, which bound the ligamental 

 cavity. This triangle is bordered on each side by a deep striated 

 cavity, and there follow two strong divergent lateral teeth, which 

 in the adult state are finely crenulated on their inner margins, 

 being covered with regular sharp transverse striee, which it is 

 rare to meet with in good preservation. 



The adductor scar which is situated against the posterior 

 margin and at a little distance from the hinge, is oval, 

 transverse, strongly marked, and rarely depressed. The pallial 

 sinus is simple, and follows the external contour at a distance 

 of from 12 to 15 mm. ; coalescing with the muscular impression 

 on the left, and forming a contour which embraces the whole 

 of the scar. The hinge plate in each valve is transversely 

 striated. The crenulation of the teeth is not seen in the young 

 state; one of my specimens shews these markings, and is 

 therefore a full-grown individual. The left valve in one 

 specimen also indicates the possession of colour markings, of a 



dark violet tint. 



r length 80 milHmetres. 

 The dimensions of our specimens are, -j^ ]gj,g° ^^j^ 55 



GEOLOGICAL POSITION, 

 &c. 



P. Icevigata is found both in France and England on the same 

 geological horizon, namely, in those upper beds of the Middle 

 Lias, designated by Quenstedt, Lias S (lower part), and 

 otherwise generally known as the Margaritatus zone of the M. 

 Lias. It is recorded with certainty as only having been obtained 

 hitherto from Churchdown near Gloucester, where it was 

 found last year by the author. One specimen of the left or free 

 valve has been presented by him to the Museum of Geology in 

 Jermyn St., London ; the other, the right or adherent valve, he 

 retains ; and would have pleasure in shewing it to anyone who 

 takes an interest in the subject. 



