m 



A. RHOMBEA. Marked for me by J. E. Gray, Esq. of the 

 British museum. It is the largest of the British arcae that 

 has come under my notice, measuring in its longest dia- 

 meter one inch and four-tenths, and in breadth nine-tenths 

 of an inch, figure of the rhomb across the hinge one inch 

 and one-tenth; surface of the valves as if worn, by the fric- 

 tion of opening in the cavity of the stone in which it was 

 included, and hence the vestiges of striae were but few. 

 About the middle of the free edges of the valves is a 

 vacancy rounded off into a long oval; this portion, when 

 the valves are closed being rilled by a semicalcareous oper- 

 culum formed on the summit of the tongue or protrusive 

 organ. A border, of regular form and about one-tenth 

 of an inch wide, circles the free margin of the valves, being 

 covered with short bristles in narrow lines. Some marks 

 of these hairs appear on other parts, especially on the 

 terminal angle of the opening. Colour dull yellow, bristles 

 brown. 



This, the only specimen which I have met with, was found 

 in a cavity originally formed by a Pholas in sandstone from; 

 deep water,, and was for some time alive in my possession. 



PECTUNCULUS. 



GENERIC CHARACTER: Shell orbicular, almost lenti- 

 cular, the valves equal, sides almost equal, close. Hinge 

 arched, with a series of many,, oblique entering teeth, the 

 middle ones obsolete, nearly obliterated. Ligament ex- 

 ternal. This genus is distinguished from area by its 

 orbicular form, and by the hinge being arched instead oi 

 straight. The teeth also are less numerous, more separate 

 and larger ; and they never gape. The beaks are not very 

 distant, yet are always separated by an external^ narrow,, 

 angularly furrowed, rather hollow facet, to which the 

 ligament is attached, and which distinguishes the Pec- 

 tunculi from the genus Nucula ; the ligament of which is 

 partly interior, and which has no facet between the beaks* 



* P. PILOSUS. Area P. Turt. Lin. A. Glycimeris. 

 Pen. Brit. Zo., vol. 4, pi. 58. fig. 58. A. P. Mont. Test. 

 Brit., vol. 1, p. 136. P. P. Flem. Brit. An., p. 4C(X 

 Common, and locaily abundant. 



NUCULA. 



GENERIC CHARACTER: The shell transverse, triangu- 

 larly ovate or oblong, the valves equal, sides unequal; no 

 facet between the beaks ; hinge linear, broken, many 

 toothed, interrupted in the middle by an obliquely ex- 

 tending spoonshaped pit; the teeth numerous, often pro- 

 duced as in the Pectens ; the beaks contiguous, curved 

 backwards. Ligament marginal, and partly internal, 

 inserted in the pit or *poon of the binge. 



