PYRGOMA GRANDE. 365 



tergum. Tbat portion of the scutum which corresponds with the valve 

 in ordinary cases, and alone is externally visible as long as the oper- 

 culum is united by the opercular membrane to the sheath, is narrow, 

 with the basal margin considerably hollowed out : the occludent edge 

 is formed into thick teeth. The Tergum is elongated, rather exceeding 

 in length the scutum, the latter being measured from the apex to the 

 rostral projection of the adductor plate. The surface of the valve is 

 depressed in the line of the spur, with the basal end of the latter 

 bluntly pointed. A very slight flexure (fig. 7 c) on the basal margin 

 indicates where we may believe the spur to commence, showing that it 

 rather exceeds in length the whole upper part of the valve. The lines 

 of growth obscurely indicate a tendency to the formation of a slight 

 "occludent ledge" along the carinal margin. Traces are just visible 

 of crests for the attachment of the tergal depressor muscles. 



5. Pyrgoma GRANDE. PI. 13, fig. 1 a — 1 d. 



Nobia grandis. G. B. Sowerby, junr. (sine descript.) Conclio- 



logical Manual, fig. 29, 1839 * 

 Creusia grandis. Chenu. Illust. Conch., Tab. 1, fig. 2 a, sed non, 



%. 2. 



Shell moderately convex, nearly smooth : scutum and 

 tergum calcified together ivithout any suture : scutum fur- 

 nished with a small occludent ledge, with the adductor ridge 

 descending below the basal margin : tergum square ivithout 

 a spur. 



Hab. — Singapore and East Indian Archipelago ; Mus. Brit., Cuming, Stutch- 

 bury ; imbedded in two kinds of coral. 



Appearance and Structure of Shell and Basis. — The shell is conical, 

 though to a variable degree, and sometimes is much depressed. The 

 surface is smooth, with only traces of narrow approximate ridges. The 

 colour is white, often with a tinge of dark purple. The orifice is oval, 

 and moderately large. The shell and a small portion of the basis 

 usually stand exserted above the coral. The walls are of variable thick- 

 ness ; when thick, the pores, by which they are permeated, are but 



* It is quite possible that this may be the Balamis duploconus of Lamarck, 

 but with such a character as the following, who can possibly recognise a 

 species ? "B. testa parte suprema univalvi, indivisd, convexd: inferiore turbi- 

 nate!, non clamd : aperturd ellipticd. L'exemplaire est sans opercule et in- 

 complet." 



