ORTHIDES OF THE HAMILTON GROUP. 49 



area scarcely more than half as wide as the ventral area, slightly in- 

 clined inwards. Ventral valve gibbous towards the umbo, depressed- 

 convex in the centre and flattened towards the front ; the front margin 

 straight, or without sinus ; area less than a line in width ; foramen 

 very broad, nearly twice as Avide as high ; beak slightly incurved and 

 neatly pointed. 

 Surface marked by fine, radiating, bifurcating striae, which are crossed 

 by finer concentric strias, and by more distant subimbricating lamellose 

 lines of growth. Minute tubular openings are observed upon the surface 

 of the striae in nearlj^ all conditions of preservation, sometimes giving 

 a subimbricated appearance to the lines. Besides these, well preserved 

 specimens show minute pores or puncta, distinct from the punctate 

 structure of the shell ; while the shell about the margins of these ap- 

 pears to have been extended, to form slender seta, the bases of which 

 are sometimes preserved. In weathered specimens, or in those v/here 

 the striae have been partially dissolved by the decomposition of iron 

 pyrites, these characters do not appear. 



The interior of the dorsal valve shows a prominent cardinal process, 

 which is continued in a strong median ridge for about half the length of 

 the valve, below which are sometimes seen a few vascular markings. The 

 impressions of the adductor muscles are, rarely, faintly separated by a 

 transverse undefined elevation. 



The muscular imprint in the ventral v.alve, which extends for two- 

 thirds or more of its length, is somewhat broadly oval, flabellate, and 

 deeply marked by the adductor muscular scar. The external striae usual- 

 ly mark the inner margins of the valves, and, in young shells or thin 

 specimens, are visible as far as the muscular impression. 



This species is closely allied to the 0. vanuxemij and may perhaps prove only 

 a variety of that ppecics. In authentic specimens the shape is more ovate, the 

 cardinal extremities less rounded, and the sides sloping almost directly to near 

 the middle of the shell ; the dorsal valve is more gibbous, and the umbo and 

 area are more inclined inwards, as may be seen by comparing the profiles of figs. 

 3 e and 3 k, Plate vi, with a corresponding figure 3 c on Plate vii. Tlie veutral 

 [ Paleontology IV. ] 7 



