Mr. Sowerby on two new Species of Orbicula, 321 



however ascertained, that it is found at Whitby, from a specimen 

 sent from that place to my brother by John Hogg, Esq, Jun. of 

 Leeds. 



OUBICULA. 



Sp. O. cancellata^ iesik orbiculari, vertice postico, marginali; 

 valvarum superficie lineis elevatis, confertis, radiantibus, 

 lineis incrementi elevatis decussatis ; valvae inferioris vertice 

 excentrali, laevi, depresso, sinu byssi parvo, brevi. 



Icon. Tab. nost. XI. f. 6. 



The general form of this shell is orbicular, and very flat, being 

 more gibbous near the posterior extremity : the vertex of the 

 upper valve is quite marginal, and posterior ; its surface is covered 

 with close-set elevated lines, radiating from the vertex, and which 

 are crossed by the elevated lines of growth, so that the entire 

 surface has a finely reticulated appearance : the vertex of the 

 lower valve is also nearly marginal, having at the posterior edge 

 a rather deep cavity, in which the sinus (through which the disk 

 of attachment passes) is placed : the surface of this valve is reti- 

 culated in the same manner as in the other, except near the umbo, 

 where it is smooth, and the lines of growth are not elevated, but 

 form complete rings, partly descending into the cavity above- 

 mentioned. The shell itself is extremely thin: it occurs in a 

 light brownish grey limestone, containing also remains of Tere- 

 bratulce and Coralloids, I am indebted to Dr. Bigsby for the 

 opportunity of describing this species, whixjh he brought from 

 horizontal limestone resting on Augtic trap, one mile north of 

 Montreal in Lower Canada. 



Sp. O, reflexa: testa subelliptica, postice acntiuscul^, polita; 

 valva superiore convexiuscula, vertice postico, submargi- 

 nali ; valva inferiore plana, vertice subcentrali, margine 

 reflexo; sinu byssi magno, elongate. 



Icon. Tab. nost. f. 7. 



The general form of this species is rather elliptical, a little 

 acuminated posteriorly — and it is quite smooth and polished all 

 over, being only marked with the lines of growth: the upper 

 valve is convex, its vertex is submarginal, being placed considera- 



VOL. II. X 



