364 BALANIDiE. 



which the articular ridge of the scutum abuts. The spur is central ; 

 its end is bluntly pointed. The total length of the tergum rather ex- 

 ceeds that of the scutum, the produced adductor ridge being included 

 in the latter. 



Affinities. — Under P. grande I shall make a few remarks, showing 

 that in several characters P. cancellation and grande are at opposite 

 ends of a short series, with P. conjugation intermediate between them. 



4. Pyrgoma conjugatum.* PI. 12, fig. la — 1 c. 



Shell nearly fiat, with approximate radiating ridges: 

 scutum and tergum calcified together without any suture : 

 scutum with the adductor ridge descending beloiu the basal 

 margin, and produced at the rostral end into a point : 

 tergum with the spur about as large as the tipper part of 

 valve. 



Hab. — Red Sea; Brit. IV J us. and Cuming. 



Appearance and Structure of Shell. — Shell white, or with a tinge of 

 purple ; nearly flat, with moderately prominent, narrow, approximate 

 ridges, radiating from the orifice, which is oval, rather narrow, and not 

 very small. In the largest specimens the ridges are less prominent 

 than in the figure given. The walls are thick, and not at all porose : 

 the sheath extends down almost to the base of the shell, and its lower 

 edge is closely attached to the walls : on each side, towards the carina, 

 there is a trace of a suture, and the lines of growth on the sheath are 

 here a little upturned. The basis is deeply imbedded and internally 

 furrowed ; the calcareous layer forming it, is thin. The largest speci- 

 men was *4 of an inch in the longer diameter. 



The Scuta and Terga (fig. 7 b, 7 e), in all the specimens which I have 

 seen, are calcified together, with no trace of a suture. There is, however, 

 a slight furrow, which, I believe, marks the normal line of separation 

 between the two valves; and in the following description this is 

 assumed to be the case. The Scutum has the adductor ridire greatly deve- 

 loped, so as to project below the ordinary basal margin to a distance as 

 great as the height of the valve. At the rostral end, this adductor 

 ridge or plate is produced into a point ; and at the tergal end, it is 

 blended with the articular ridge, and united to the inner face of the 



* Dr. Gray thinks this is the Pyrgoma stellata, of Chenu, ('Illust. Conch.') ; 

 it may be so ; but the figure given of the shell will do equally well or rather 

 better for the Pyrgomum dentatum of this work, and for sonic varieties 

 of P. crenatum. Without a careful description of the opercular valves, it is 

 really impossible to recognise, with any approach to certainty, the species of 

 this geuus. 



