370 CLYPEUS 



in the same individual at different periods of life; its most typical form is oblong, 

 convex anteriorly, produced and truncated posteriorly, and enlarged in the region of 

 the postero-lateral inter-ambulacra ; in others the circumference is nearly circular, and in 

 some few transversely oval ; the first form is, probably, characteristic of adult life, as the 

 elongation and truncation of the single inter-ambulacrum are markedly shown in the only 

 two large specimens I have seen of this rare species. In all the test is very flat; 

 the anterior half is gently and nearly equally convex, and the posterior half much 

 declined towards the posterior border. The ambulacral areas are narrow, the anterior one 

 most so ; the antero-lateral and postero-laterals are about the same width ; they have a 

 lanceolate form, and are composed of very narrow plates ; about three tenths of an inch 

 above the margin, the pores slightly diverge, and continue about the same width 

 apart until they approach the apical disc ; the distance between the rows of pores in 

 this species is less than in any other known Clypeus, and forms one of its diagnostic 

 characters ; in all the specimens I have examined the ambulacral areas are likewise 

 slightly elevated above the general surface of the test. The inter-ambulacral areas 

 are of unequal width, the antero-lateral pair are the narrowest, they are, however, about 

 nine times the width of the anterior single ambulacral area ; the postero-lateral pair 

 are three tenths of an inch wider than the antero-laterals, and the single inter-ambulacrum 

 is about the same width as the latter. The anal valley extends from the apical disc to 

 the posterior border; it is very narrow, with deep perpendicular sides above, Avhich 

 become shallow and expanded below; the postero-lateral' inter-ambulacra are enlarged at 

 the margin ; the single inter-ambulacrum is considerably produced, its posterior border is 

 broadly truncated and slightly deflected, within which the expanded sides of the anal 

 valley are excavated. 



The base is nearly flat ; the elevations are produced by the prominence of the 

 postero-lateral inter-ambulacra, and the deflection of the single inter-ambulacrum. The 

 small mouth-opening is excentral, and placed nearer the anterior than the posterior border; 

 the peristome is surrounded by five small oral lobes, which make inconsiderable promi- 

 nences at the base. The apical disc is small, and absent in most of my specimens ; in 

 one only is it preserved. The genital plates are nearly equal sized ; the anterior and 

 posterior pair are perforated, and the single plate is imperforate. The madreporiform 

 tubercle rests on the right anterior plate, and extends into the middle of the disc. The 

 ocular plates are small, and firmly wedged between the disc and summits of ambulacra ; 

 the eyeholes are large, and, with the four genital holes, form a cu'cle of perforations 

 around the circumference of the disc. 



4fflnities and differences. — Cli/peus Michelini, in its oblong form, truncated posterior 

 border, and narrow anal valley, resembles C. Miilleri, Wr. ; but is readily distinguished 

 from the latter by the form, narrowness, and structure of the ambulacral areas ; in C. 

 Miilleri they are expanded and petaloid, and in C. Michelini they arc narrow and lanceo- 



