ACASTA LAEVIGATA. 315 



fig. 5 b, representing the lateral and carino-lateral compartments, and 

 part of the carina), the six sutures are seen to be strengthened by six 

 double columns. 



Basis. — This is moderately cup-formed. The edge, in order to meet 

 the basal points of the inwardly projecting lateral margins of the six 

 compartments, has six knob-like teeth. These are placed at unequal 

 distances, for two on each side stand near each other, owing to the 

 narrowness of the carino-lateral compartments, The degree of their 

 development varies extremely ; when most developed, as in the speci- 

 men figured (PL 9, fig. 5 a), each tooth is bifid and a little hollowed 

 out, so as to receive the points of the two inwardly projecting margins 

 which form each suture. Ridges, more or less prominent, running from 

 each of the six marginal teeth, extend towards the centre of the cup. These 

 six teeth cannot be seen from the outside. The edge of the cup is rarely 

 crenated ; but I have seen two instances in which this was the case. 



Cirri. — In the first pair, the rami are not quite so unequal as in A. 

 spongites ; the longer ramus being about twice as long as the shorter. 

 In the third pair, there are some very minute, thick, upwardly-pointing 

 spines, which I did not notice in A. spongites. In the fourth pair, the 

 spines are a little more crowded, with longer dorsal tufts, than in the 

 sixth pair ; and they are mingled with some very minute, thick, up- 

 wardly pointing spines. In young individuals, there are only three pairs 

 of main spines on the segments of the sixth pair, instead of four pairs. 



6. Acasta laevigata. PL 9, fig. 6 a, 6 b. 



Acasta laevigata. /. E. Gray (!) Annals of Philosophy, (new 



series), vol. 10, Aug. 1825. 



Parietes internally quite smooth, tvith the lateral margins 

 of each compartment inwardly prominent: basis tvith the 

 edge strongly crenated, and furnished with six inwardly 

 prominent teeth ; scutum feebly striated longitudinally, or 

 smooth. 



Var. («), epidermis coloured dull orange. — Red Sea. 



Hob. — Red Sea, Philippine Archipelago; Mus. Brit., Cuming, &c. 



This species, of which I have examined many specimens 

 from the above two and other unknown localities, agrees in 

 all essential points of structure with A. glans, and conse- 

 quently I for some time classed them together; but the 

 characters, though usually of small value, by which this 

 form differs from A. glans being apparently constant, I 



