226 BALAN1DJ1. 



margin on the carinal side forms a distinct but obtuse angle with the 

 spur. In all cases the crests for the depressor muscles are very feebly 

 developed. 



The Compartments have their radii developed to a rather varying 

 degree, with their summits oblique ; hence the orifice is toothed : the 

 sutural edges of the radii have their septa barely denticulated ; the 

 sutural edges of the alse are smooth. The basis, as with the other 

 species of this section, is permeated by pores ; yet I found one speci- 

 men, from the Cape of Good Hope, with the basis apparently solid, 

 thus offering a very singular anomaly. In the specimen imbedded in 

 sponge, the basis, as viewed externally, is concave ; whereas in Acasta, 

 which always inhabits sponges, the basis is highly convex or hemi- 

 spherical. 



The Mouth and Cirri resemble those of B. trigonus, and I can point 

 out no distinguishing character. 



With respect to the variety from the West Indies, I have seen two 

 sets of specimens differing somewhat in external appearance, one set 

 attached to a coral from St. Vincent's, and another set to an Avicula 

 from an unknown locality ; at first I described these specimens, with 

 some hesitation, as a distinct species, and I am very far from sure 

 whether this would not have been the more correct course, although I 

 am unable to point out any sufficient diagnostic characters. This form 

 differs from the ordinary B. spongicola, in the walls being more rugged, 

 stronger, and slightly or deeply folded longitudinally; in this latter 

 case the shell in external aspect differs much from ordinary specimens 

 of B. spongicola ; but this is a variation so common that I dare not 

 place any reliance on it. The colour is more purple ; the summits of 

 the radii perhaps rather less oblique. In the scuta the only difference 

 is that the articular ridge seems rather longer, and the adductor ridge 

 perhaps more prominent: in the terga, as already remarked, the basal 

 margin on the carinal side does not slope so straight into the spur. 

 These differences I consider all too slight to be of specific value. The 

 difficulty in determining the nature of this variety is added to by its 

 approach to B. trigonus in all those points in which it departs from the 

 ordinary B. spongicola, so that for a short time I was even tempted 

 to consider both these species as varieties of one form. But until 

 B. trigonus is found with its scutum longitudinally striated, and with 

 its tergum beaked, it can hardly be confounded with B. spongicola ; 

 for it should be observed that when in B. trigonus the rows of little 

 pits disappear from the scuta, as sometimes happens, though rarely, 

 yet these valves do not become longitudinally striated. 



Bat anus spongicola occurs, mingled with B. tulipiformis, in the 

 Mediterranean, and by the external characters of the shell alone cannot 

 be distinguished from that species ; but the striated scuta and beaked 

 terga suffice to separate them. Again, this species, at the Cape of 

 Good Hope, occurs mingled with J?. Capensis, and from the non-striped 

 young varieties of that species, it can, externally, be distinguished only 

 by the beak of the tergum not being sharp like a needle. I have seen 

 a single, perfectly characterised specimen, with its opercular valves 

 preserved, found by Mr. S. Wood in the Coralline Crag at Sutton, 

 mingled with B. inclusus. 



