PYRGOMA DENTATUM. 369 



7. PYRGOMA DENTATUM. PL 13, fig. 3 a — Sg. 



Scutum much elongated, with a tooth-like articular pro- 

 jection : tergum convex, irregularly triangular, sometimes 

 with an imperfect spur, and on the internal surface with an 

 inwardly projecting tooth ; scutum and tergum furnished with 

 an occludent ledge. 



Var. (1), 3 c, 3g: tergum, with a sharp internal tooth, projecting 

 rectangularly inwards. 



Var. (2), 3 d : tergum, with a broad blunt internal tooth, depending 

 beneath the spur-like portion of the valve. 



Var. (3), 3f: tergum, with the basi-carinal end of the valve truncated, 

 with a small blunt internal tooth projecting rectangularly inwards. 



Hab. — Red Sea; Mus. Brit, and Cuming. Also associated with Vyrgoma 

 crenatum, and attached to Meatidrina spongiosa.* 



Appearance and Structure of Shell. — Shell nearly flat, oval, white 

 or pink, with rather distant prominent ridges radiating from the mode- 

 rately large (for the genus) oval orifice. The ridges are often obscured, 

 and apparently sometimes almost obliterated by the incrusting coral. 

 Shell permeated near the outer lamina by short imperfect pores : in- 

 ternal surface smooth : sheath inconspicuous, descending rather more 

 than half way down the walls ; lower edge closely attached to the walls. 

 Basis deep. Diameter of largest specimen '3 of an inch. 



Scuta : these are elongated, but to a very variable degree, some 

 specimens being quite three times, and some barely twice as broad as 

 high. I observed this same variability in two sets of specimens, dif- 

 fering, as we shall presently see, in the form of their terga : it depends 

 in part, but not wholly, on the varying width of the occludent ledge, 

 which is sometimes only a fourth, and sometimes half as high as the 

 rest of the valve. This is the first species in the genus in which the 

 occludent ledge — a structure peculiar to the genus — has been amply 

 developed. The basal margin of the valve is slightly sinuous, and a very 

 little hollowed out near the basi-tergal corner; it is also very slightly 

 reflexed, the reflexed portion being separated from the upper part of the 

 valve by a very slight depression or even furrow. I notice this slightly 

 reflexed portion, simply as indicating a well marked feature in the basal 

 margin of the following closely allied species. Internally (3 g), the ad- 

 ductor ridge is thick and slightly prominent, but does not descend 



* Mr. Dana informs me that he believes that this coral comes from the 

 West Indies ; though the specimens originally described by him had no label. 

 If this be so, both Pyrgoma dentatum and crenatum have verv wide ranges. 



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