458 BALANIDiE. 



with a white edge to the lips lining the aperture between the opercular 

 valves. 



Structure of Shell and Radii, fye. — The under side of the parietes is 

 either quite smooth, or marked with slight, branching, depressed 

 lines; or mamillated ; or irregularly studded with large pores. I have 

 already alluded to the pillars, depending from the under sides of the 

 aloe in most specimens of var. depressus : these pillars tend to form 

 ridges, parallel to the sides of the compartment, like those we shall 

 presently see in certain specimens of C. scabrosus. The radii are very 

 narrow, when best developed ; their edges, when disarticulated, can be 

 seen, when examined by a strong lens, to be finely crenated. The 

 edges of the alae are likewise very finely crenated. 



Scuta. — The outline of these valves varies considerably in specimens 

 taken out of the same group : we have either a nearly equilateral triangle 

 (fig. 1/) or the tergal margin (1 e) is much shorter than the other margins. 

 There is always a deep depression for the adductor muscle, and a small 

 pit of very variable depth for the lateral depressor. But the tergal 

 margin offers the greatest variability ; here we see a very prominent 

 articular ridge or fold, having either a straight edge or a single or 

 double prominence (fig. 1 e — 1 h). In specimens in the same group we 

 find considerable variation in these points ; but the amount of difference 

 is sometimes so great, that I long hesitated whether to rank some of 

 the varieties as species. The Terga, likewise, vary greatly in shape 

 and width : in some of the commonest varieties (and in var. depressus) 

 the valve is very narrow, with the under surface channelled or concave : 

 in other varieties the valve is much broader and flatter. The spur is 

 but slightly developed. The crests for the depressor muscles barely 

 descend beneath the basal margin of the valve. The articular ridge, in 

 some varieties (as in specimens from Madeira and the Cape de Verde 

 Islands), is extremely prominent and straight (1/) ; in others, it is little 

 prominent and deeply sinuous(l h). This great variabilityin the articular 

 margins of the scutum and tergum seems to be mainly due to the cor- 

 rosion to which these valves have been subjected, and their consequently 

 modified growth : in some specimens the articular ridge of the scutum, 

 and in others that of the tergum, has been largely developed, in either 

 case their mutual outlines being greatly affected. 



Branchice. — These are narrow, hardly at all plicated, elongated, 

 being about half as long as the sack. In a specimen from La Plata, 

 this organ ended rather more abruptly in a point than it did in other 

 specimens. 



Mouth. — The crest of the labrum is usually hairy, but in a specimen 

 from Bahia (Brazil) there were some very fine teeth. The palpi vary 

 somewhat in shape, and sometimes have a row of bristles along their 

 basal exterior margins. The mandibles usually have four main teeth, 

 the lowest one being confluent with the inferior pectinated angle : in 

 var. depressus, in the same individual, there were only three teeth on 

 one side of the mouth and five on the other ; the lower main teeth are 

 laterally double, but generally one tooth of each pair is so small and 

 obscure as to be perceived with difficulty. In the maxillae, there are 

 some large spines above the notch, and in the notch some fine ones : 



