1879.] 



MR. F. J. BELL ON THE ECHINOIDEA. 



2.55 



tinal boundary of the ambitus, and is never closed ; in M. grandis it 

 forms a slightly convex line, vrhich bounds the posterior end of the 

 actiual plastron, and then turns upwards at a very open angle ; in 

 M. ventricosa the horizontal line is straighter, the lateral bauds 

 longer and almost perpendicular to the former. In Brissus the 

 same fascicle is a little broader, amd is always closed, the resulting 

 figure bein2: cordiform, often more or less truncated at its base. In 

 Metalia the fasciole around the subanal plastron is still broader ; 

 and there is, in addition, a narrower band on either side of the anus, 

 which extends just beyond the ambitus, and is of the form of the 

 fasciole in Meoma ventricosa : the result is, that we have the effect 

 of the presence of the fascioles of both Oleoma and Brissus in Me- 

 talia. Whatever value these characters have from a genetic point 

 of view, there can be no doubt as to their ready accessibility and 

 general constancy. 



To resume. Meoma displays the simplest form of subanal fasciole, 

 and the most general distribution of the primary tubercles ; the 

 anterior ambulacrum is but slightly depressed; and the anterior 

 lateral ambulacra are to the posterior ones in the proportion of from 



1000 . ]000_ 

 lOSl ^^ 1135* 



In Brissus the subanal fasciole is closed, and the larger tubercles 

 are absent from the posterior portion of the abactinal surface ; it is 

 rarely that the anterior ambulacrum is depressed beyond the level of 

 the test ; and the lateral ambulacra stand to one another in the propor- 

 tion of from ^ to \^ (in adult specimens the ratio barely exceeds 



j^)- The subanal plastron is provided with three or four pores 

 on either side, but there are no radiating bands. 



It is in Metalia only that the anterior ambulacrum is ever found 

 in a deep and well-marked groove ; the larger tubercles are confined 

 •within the peripetalous fasciole ; the subanal fasciole gives off bands 

 to either side of the anus; and the lateral ambulacra are to one 

 another in ratios varying from y,-^(, to ijjo'* "^^^^ subanal plastron may 

 have as many as nine pores on either side ; and well-marked radi- 

 ating lines extend outwards to them from the more median region 

 of the plastron. 



