BRITISH FOSSILS. 



Decade III. Plate III. 



TEOPIDASTER PECTINATUS. 



[Genus TROPIDASTER (new.) Forbes (1850.) (r^^y*/?, a keel, and MT-no, a star.) 

 (Sub-kingdom Radiata. Class Ecliinodermata. Order AsteriadfE. Family Asterite.) Body 

 stellate, five-rayed [a vent on the dorsal surface?] ; rays convex and caritiated above, the 

 carina composed of a double series of squamose plates ; rest of the dorsal surface spinous, 

 spines simple ; ambulacra bordered by transverse plates with spiniferous crests on their 

 anterior margins ; ambulacral ossicula rather broad, geniculated, pectinated at their 

 inner extremities. (Suckers biserial.)] 



Species TJnica. Tropidaster pectinatus. 



Description. — The general aspect of this star-fish when seen from 

 above is that of a Uraster^ whilst viewed from below it resembles an 

 Astropeden. The rays are rather short, about equal in length to the 

 breadth of the disk, ovato- triangular. The upper surface of the rays 

 and disk is covered with short obtuse simple spines, which, on the 

 sides of the rays, are ranged in oblique rows of about five or six in each 

 row. Placed rather laterally on the disc is seen, though obscurely, a 

 madreporiform plate, and 1 think I can perceive indications of an anal 

 pore. Down the centre of each ray runs a keel composed of two rows 

 of squamous plates, somewhat quadrate in form, but produced at their 

 anterior and inner angles. This kind of keel, or mid-rib, is not 

 present in any star-fish, recent or fossil, with which I am acquainted, 

 and resembles in form and structure the tiling of the crest of the roof 

 of a house. It must be regarded as a peculiarity of generic value. 

 The rays themselves appear to have been very flexible, and not much 

 liable to injury. Their extremities are rather pointed. On the under 

 surface their centres are occupied by the rather broad lanceolate 

 ambulacral grooves running from the mouth ; that orifice is some- 

 what contracted by the encroachment of the large twin triangular 

 plates with punctated surfaces which occupy the angles formed by 

 the junction of the bases of the rays. The ambulacral ossicula are 

 oblong, rather broad, strongly geniculated in the centre at their 

 anterior margins, and denticulated by about five crenations at the edge 

 which borders the ambulacral sulcus. The margins of the under 

 surfaces of the rays are bordered by transverse, oblong, rather nan-ow 



[ill. iii.] B 



