ACASTA SPONGITES. 309 



General Appearance. — The shape is usually that of a cup, the orifice 

 being, in most cases, rather large, and deeply notched, owing to the 

 great obliquity of the radii and alee. The surface is generally smooth, 

 but furnished with sharp calcareous projections. The colour is 

 pinkish, and chiefly in the upper part of the shell ; the lower part is 

 often yellowish from the preserved epidermis. The parietes in the 

 carino-lateral compartments are always narrow, being only one sixth or 

 one seventh of the width of the parietes in the lateral compartments. 

 The radii are not very wide, never equalling in width the parietes. 

 The basis is moderately deep, and sometimes very deep, being even occa- 

 sionally curled like a horn on one side. The specimens from Lisbon and 

 Naples are a little larger than any British specimen which I have 

 seen ; the former being '3 of an inch in basal diameter. 



Scuta: these are striated longitudinally in close lines, generally 

 plainly, but to a variable degree. The whole valve is flat, thin, and rather 

 elongated, with barely a trace of an adductor ridge : the articular 

 ridge is short and rather prominent: it terminates downwards abruptly, 

 and this does not appear to be the case in the two following species. 

 Terga: these are small compared with the scuta, they are slightly 

 beaked : the spur is truncated but rounded, more especially on the 

 carinal side ; it is rather more than one third of the width of the 

 whole valve. The articular ridge and crests for the depressores muscles 

 are feebly developed. 



Structure of the Parietes and Radii. — The state of the inner surface 

 of the parietes varies much ; generally they are slightly ribbed close to 

 the basis, the ribs sometimes extending up to the sheath ; rarely the 

 surface is quite smooth. The edges of the radii are slightly crenated. 

 The upper internal surfaces of the radii, where overlying the alse, are 

 usually marked by feeble undulating lines, nearly parallel to the basis. 

 The alse have very oblique summits. 



Basis: this is generally of a regular cup-like form, and about two 

 thirds as deep as the shell is high ; sometimes it is pointed at the 

 bottom and distorted. The edge is feebly crenated, and rarely quite 

 smooth. It is often penetrated by small rounded irregular holes ; and 

 I have seen specimens from the Cape of Good Hope with parts like a 

 sieve. 



Cirri: in the first pair, the anterior ramus is nearly thrice as long as 

 the posterior ramus. The second cirrus is short, with one ramus 

 longer by three or four segments than the other ramus ; the terminal 

 segments are truncated. The third cirrus is about one third longer 

 than the second cirrus. In the anterior ramus of the fourth pair, the 

 regular pairs of spines are rather crowded together in the upper part of 

 each segment, and the intermediate little spines and dorsal tufts are 

 rather long in comparison with those of the fifth and sixth pairs of 

 cirri : moreover, amongst the regular pairs, a few very minute and 

 thick spines, pointing upwards, could be perceived. So that we here 

 have the very unusual case of the fourth cirrus not exactly resembling 

 the fifth and sixth pairs ; and we shall see, in the following species, 

 that this same anterior ramus of the fourth cirrus presents in addition 

 another very extraordinary character. In the sixth cirrus there are on 

 each segment four pairs of spines. 



