2 



BRITISH FOSSILS. 



" The greatest width of the body is nearly on a line with the 

 termination of the postero-lateral ambulacra. The tubercles of the 

 back are numerous and closely set, and increase gradually in size 

 in the anteal region, and towards the apex. The lateral ambidacra 

 are narroAv, somewhat linear in sha.pe, and deeply impressed, 

 showing on the surface as four radiating furrows, two of which, the 

 antero-lateral ones, stand at right angles to the longitudinal 

 diameter of the test, whilst the other two, the postero-laterals, are 

 directed obliquely backwards, and form with one another an acute 

 angle at their apical terminations. The posterior pair of ambulrcra 

 are a little longer than the others, and contain rather more pairs of 

 pores, the respective numbers in each row being about 27*30 or oO'S5. 



" The centro-ambulacral space is smooth, or nearly so, in the lateral 

 ambulacra, but in the odd or anteal ambulacrum, which, instead of 

 being impressed and subpetaloid, is linear and plain, or even slightly 

 elevated.* It is regularly and minutely granulated, the large granules 

 or small tubercles forming boundary rows. In the lateral ambulacra 

 the ridges separating the pairs of pores are minutely granulated. 



" The genital disk, usually obscured in fossil specimens, has four 

 genital holes, the two posterior ones largest, and five eye perfora- 

 tions of a peculiar structure. 



" The peripetal fasciole is very distinctly marked. In front of the 

 antero-lateral ambulacra it includes a wide somewhat semicircular 

 space, its foremost and central portion crossing the shell at a little 

 ],>elow half its height. From this point tracing its course along 

 each side, it runs with a slight angularity to about two thirds of the 

 distance between the anteal ambulacrum and the end of the antero- 

 lateral one, before meeting which it makes a single strongly marked 

 incurved flexure, in this respect differing from Brissus carinatus, 

 the fasciole of which has two inflexions in this region. It then 

 winds closelv round the end of the antero-lateral ambulacrum, and 

 ascends rapidly between it and the postero-lateral one, making a 

 deep but wide flexure, somewhat truncated at its upper part, then 

 curves down, following closely the bounds of the postero-lateral 

 ambulacra, round which it winds and crosses the posterior inter- 



''^ In this fossil specimen, much more so than in any of the recent ones in the British 

 Museum. The carination of the back, too, is considerably more marked. 



Is there really any difference between the Jj. carinatus. Lam, from the Mauritius, and 

 this Mediterranean species ? The double angle of the fasciole is not always constant in 

 B. carinatus, or absent in B, Sc'dlce. The overhanging of the posterior end in the former 

 is a better mark of difference, but a specimen in the British Museum from the Mauritius 

 has it nearly perpendicular. The granulation, under surface, and ambulacra are very 

 much the same in both. — J. W. S. 



