2 



BRITISH FOSSILS. 



ally eniarging towards the mouth, instead of being comparatively few 

 and suddenly becoming large and scattered in that region. Its body is 

 also more rounded and tumid at the margin, although that character 

 varies slightly according to the degree of convexity of the dorsal sur- 

 face. 



The only Holectyjpus described in the Monograph of the Galerites by 

 Desor, with a marginal anus, is the H. hemisphcEricus, and the figures 

 and description given by that author are sufficiently characteristic of the 

 British gpecies now under examination, so that I do not hesitate to adopt 

 Desor's name. Its general outline is sub-hemispherical, varying in con- 

 vexity in different individuals. Specimens are rarely quite regularly 

 convex, but most commonly have a slight obliquity, and are very 

 slightly elongated and declining on the side towards the anus ; conse- 

 quently the vertex in such examples is not quite central, though con- 

 stantly apical.* The sides are gently tumid, so as to round off the 

 margins gradually. The ventral surface is more or less hollowed out 

 in the centre, often very much so. In the depth of the concavity, and 

 nearly or quite central, is the mouth, occupying less than one-third of 

 the disk, round, with ten deep marginated notches separated by gently 

 curved interspaces, and placed close to the junctions of the interambu- 

 lacral areas with the ambulacral avenues. The ambulacral areas are a 

 third or more of the breadth of the interambulacrals, their proportionate 

 dimensions increasing generally with the altitude of the individual, and 

 lessening usually with age and size. The pairs of pores of the avenues 

 are ranged in single file ; they are slightly more distant from each 

 other on the under than on the upper surface, but those very near the 

 mouth are closely set again. There are about four pairs, or four and 

 part of a fifth, opposite each interambulacral plate towards the middle 

 of the upper sides. In a specimen slightly above an inch in diameter, 1 

 counted twenty- six interambulacral plates in a series, and ninety-four 

 ambulacrals, each of the latter representing a pair of pores. Each of 

 the interambulacral plates is thickly studded with minute granules, in the 

 midst of which are the spiniferous tubercles, scattered and varying in 

 number in different individuals. I have found some specimens, closely 

 alike in all other respects, varying in the number and arrangement of 

 these tubercles, of which there are sometimes two rows on a plate, at 

 others only one. In those examples which present the most regular 

 arrangements, the tubercles are so disposed on the assemblage of inter- 

 ambulacral plates, that each half area exhibits a curved line of them 

 rising to alternate at the suture with a similar curved line on the 

 corresponding half area. On the ambulacral areas the tubercles 



* I use the term " vertex" to denote the highest point of the back, and apex" to indi- 

 cate that point to which the ambulacra converge and where we find the ovarian disk. 



