96 Dr. T. Wright on Fossil Echinoderms 



Hemiaster Cotteauii, Wright. PI. VII. fig. 2a-d. 



Test orbicular, globose, much inflated, declining anteriorly, ele- 

 vated posteriorly, the interambulacrum forming a prominent 

 carina which terminates in a tail-like process above the anus ; 

 posterior border obliquely truncated ; ambulacral areas deeply 

 sunk ; an anteal ambulacrum forms the sulcus in the anterior 

 border; antcro-laterals long, and inclined to 45; postero- 

 laterals one-half the length of the anterior pair, inclined to 

 57; apical disc nearly central; peripetal fasciole broad and 

 undulating ; anus high under the carinal process ; tubercles 

 larger on the sides and base than on the dorsal surface; 

 mouth labiate near the anterior border. 



Dimensions. Antero-posterior diameter 1^ inch, transverse 

 diameter l^g inch, height l|i inch. 



Description. This Urchin has a globose form, and is much 

 inflated at the sides ; the dorsal surface is convex, and declines 

 much more rapidly from the apical disc to the anterior border, 

 than from the disc to the posterior border. The ambulacral 

 areas (2 a) are all deeply sunk ; the single ambulacrum is the 

 longest, and forms a considerable anteal sulcus; the antero-lateral 

 pair have a gentle double curve ; they are T 7 oths of an inch in 

 length, and form an angle of 45. The number of pores (2 c) 

 in the avenues is twenty-two pairs in the inner, and twenty-four 

 in the outer zone ; the postero -lateral pair are scarcely half the 

 length of the anterior pair; they incline at 57; their number of 

 pores is ten and twelve pairs. The peripetal fasciole (2 d) closely 

 embraces the ambulacral star ; a naked track proceeds from the 

 base of the antero-laterals to the mouth, indicating the course 

 of the imperforate portion of the ambulacral areas : the rapid 

 declivity of the anterior part of the test strongly contrasts with 

 the inflated condition of the sides and the elevation of the in- 

 terambulacrum ; from the centre of this area a ridge rises which 

 is produced into a tail-like process, and beneath, the posterior 

 border is scooped out, and truncated obliquely downwards and 

 inwards. The anus is situated high up, immediately beneath 

 the caudal prolongation ; the base is convex, and a partially 

 naked space on each side of the sternal portion of the inter- 

 ambulacrum, indicates the track of the basal portions of the 

 posterior ambulacra. The tubercles of the upper surface (2 b) 

 are smaller and more closely set together than those on the 

 sides and base, where they are larger, wider apart, and more 

 fully developed. They are perforated and uucrenulated, and 

 surrounded by a circle of small tubercles. H. Cotteauii resembles 

 Spatangus (Hemiaster) acuminatus, Goldf., but it is more globose, 

 and its posterior half is neither so elevated, nor yet so wedge- 



