PYRGOMA CANCELLATUM. 363 



Hab. — Imbedded in a Gernrnipora, probably from the West Indies ;* Mus. 

 Brit, and Cuming. 



Appearance and Structure of Shell. — Shell nearly flat, sometimes 

 tinted dull dirty purple, with the surface marked by slight, broad, 

 approximate ridges, the ends of which form considerable projections, 

 giving to the shell, when not too much encrusted by coral, a lobed 

 border. In young specimens (-15 of an inch in diameter) some of the 

 points projected half as much as the semi-diameter of the shell, giving 

 it a radiating appearance. The orifice is oval and rather small, but of 

 variable size. The shell is thick, and, near the outer lamina, is pene- 

 trated by pores : the internal surface is smooth. The sheath (fig. 5 b) 

 extends down but a short distance from the orifice ; it is closely attached 

 to the walls : the lines of growth, at a point on each side, bend a little 

 downwards (instead of upwards, as in the two following species), and 

 hence the lower edge of the sheath irregularly projects downwards on 

 each side. The basal cup, internally, is plicated, the hollows corre- 

 sponding with the projecting, longitudinal, parietal septa, which form 

 the lobed border of the shell. The largest specimen which I have seen, 

 was rather above '4 of an inch in diameter. 



The Scuta and Terga are not calcified together : they are both much 

 elongated. Scutum : (fig. 5 c, be) for the first time in the genus, or 

 indeed in the family, this valve presents a remarkable character in the 

 adductor ridge being immensely developed, so as to project far below 

 the ordinary basal margin. At the rostral end, it at first appears to pro- 

 ject even more than it really does, for the toothed occludent margin is 

 in fact a prolongation of the true valve, as distinct from the adductor 

 plate. Excluding this very narrow, prolonged, occludent margin, the 

 adductor plate projects for a length equalling the rest of the valve. 

 Along the tergal margin, the adductor plate is united to the articular 

 ridge ; and at the rostral end, it is produced into a square tooth, whence 

 a square-edged ridge extends on the surface of the plate upwards to the 

 ordinary basal margin of the valve. The exact shape of this adductor 

 plate varies a little, as does the degree to which it is closely attached 

 to the ordinary basal edge of the valve. The valve, as distinct from 

 the adductor plate, is narrow, with the basal margin regularly curved. 

 The articular ridge is very prominent. 



Terga extremely narrow, linear, consisting in chief part of the spur, 

 which is fully four fifths of the entire length. Externally (fig. 5 d) the 

 valve is furrowed, with the edges more or less folded in along the spur. 

 The upper or ordinary part of the valve is about one third wider than 

 the spur. The basi-carinal angle is sharp, owing to the basal margin 

 being a little hollowed out. A special plate of shell (fig. 5/), hollow 

 under its basal edge, runs from the carinal margin to the articular 

 ridge, which latter is situated in the middle of the valve, and against 



* In this case I am again indebted to Mr. Dana for naming the coral ; he 

 informs me that the genus is found in the Pacific and East Indies ; the specimen 

 sent he believes is the G. cinerascens. 



