ACASTA UNDULATA. 313 



Cirri : these resemble, in every respect, those of A. spongites, with 

 the remarkable exception that on the anterior ramus of the fourth 

 cirrus, several segments were furnished with the beautiful downward 

 curved, mandible-like teeth, as in A. sulcata ; but differently from 

 in that species, there were none on the upper segment of the pedicel. 

 1 should have thought this an excellent specific character, had not these 

 teeth been so extremely variable in A. sulcata. 



Finally, I think this species is more nearly related to A. sulcata 

 than to A. spongites. 



4. ACASTA UNDULATA. PI. 9, fig. 4. 



Shell, apparently, as in A. spongites, but larger : scutum 

 marked by longitudinal ridges, often in pairs, with the inter- 

 mediate furrows rather wide : spur of tergum nearly half as 

 wide as valve. 



Fossil in Coralline Crag (Sutton), Mus. S. Wood. 



I owe to Mr. Wood the inspection of a fine suite of 

 separate valves. Owing to the shell never having been 

 found entire, its general shape is not known, and, what is 

 of more consequence, the relative proportional width of the 

 parietes of the carino-lateral compartment is unknown. I 

 have (but with doubt) given it a distinct specific name, 

 owing to the peculiar character of the furrows on the scuta, 

 and to the large size of the whole shell. In its other 

 characters it comes nearest to A. spongites, excepting in the 

 spur of the tergum, which resembles that of A. sulcata. 



The compartments appear to have been rather smooth externally. 

 The radii are not wide, as in A. cyathus ; and the basis is cup-formed. 

 Internally, the parietes are feebly ribbed, as in A. spongites. 

 Judging from the dimensions of the separated valves, this species must 

 have equalled and perhaps exceeded the size of the largest living 

 species, namely, A. glans, from Australia. Hence we may infer, that 

 the basal diameter probably exceeded '55 of an inch : I may add, that 

 the largest European specimens of A. spongites, from Naples and 

 Portugal, are only *3 of an inch in basal diameter. 



Scuta. — These seem to resemble the scuta of A. spongites in all 

 respects, except in the longitudinal ridges standing much further apart, 

 and, consequently, in the furrows being much wider: each ridge is 

 generally double. Although there is a good deal of variability in the 

 character of these ridges iu A. undulata, and likewise in A. spongites, 



