Classijicatory Position of Hemiaster elongatus. 229 



understood on reading Lovdn's summary of the apical struc- 

 tures {o^). cit. p. 79) — " Palcnostoma offers a calicinal system, . . . 

 . , . with the five radials [ocuLar plates] distinct, the I. and V. 

 widely separated [that is, the postero-lateral ocular plates], 

 and out of the costals [generatives, or the basals of one of us], 

 the 3 alone defined by a suture [the left anterior], all the 

 rest being coalesced into one piece ; with the madreporic 

 filter represented, in the young specimens examined, by a few 

 punctures placed before the middle, and with the two huge 

 sexual outlets, mammiform and prominent, occupying a con- 

 siderable portion of the system and placed transversely 

 against the interradials 1 and 4, so as to prevent the retro- 

 grade passage of the madreporite." In other words, the 

 madreporite does not pass backwards between the postero- 

 lateral generative plates Nos. 1 and 4. 



Tlie ])eristome of Palceostoma is very excentric in front, is 

 small, flush with tlie test, and of course is five-sided, with the 

 five angles well pronounced. The sides of the pentagon are 

 nearly equal in length, and nearly the whole of each is formed 

 by the margin of a corresponding interradial plate. One 

 cannot but notice the width of the inner (peristomial) part of 

 the interradial plates which bound tlie mouth, and that whilst 

 these plates occupy so much of the peristomial region, the 

 inner or peristomial plates of the ambulacra are at the angles 

 of the mouth, and hardly enter into the construction of the 

 orifice at all. Broad as the ambulacral plates certainly are, 

 they only touch the peristome with their tops, so that whilst 

 the ambulacra I. and V. form points at the angles of the base 

 of the pentagon, the ambulacra 11. , III., IV. only form 

 blunter points at their correspraiding angles. At the lowest 

 computation the interradial plate No. -/ of each interradium 

 occupies two thirds of that side of the mouth, and the ambu- 

 lacral plates " a " 1 and " h " 1 only the remaining third. 



The peristome is angular in front and has a straight broad 

 margin posteriorly, the postero-lateral sides of the mouth 

 are of the same breadth as the posterior margin, and the 

 remaining edges of the mouth are equal in dimensions to the 

 others already noticed. It is very important to realize this 

 preponderance of size of the interradial plates No. / over 

 those of Nos. " a " 7 and " Z) " / of the ambulacra. This ar- 

 rangement is invariable in Palceostoma. 



There is no downward projecting lip to the plate 1 of the 

 odd interradium, there is no arched margin in front of the 

 mouth, joining the sides of the lip with a narrow curve, and 

 the lower edges of the peristomial marginal plates are not 

 rounded off or otherwise ornamented ; they are sunken. 



