504 CLYPEASTRIDAE. 



CLYPEASTRIDAE. 



Suborder Clypeastridae Agass , 183G, Prod. Mon. Rad. 



The suborder of Clypeastridae has been recognized much within the same 

 limits as first established by Agassiz. I am fully aware that the tendency 



<>f all recent writers on Echini has been to adopt, as most natural to the 

 presenl condition of our knowledge, the division into two suborders, as pro- 

 posed by Albin (iras. As 1 have already shown in my Embryology of 

 the Star-fish, and from the more recent study of young Echini in the pre- 

 liminary Report of the Deep-Sea Florida Echini, the separation of the anal 

 opening from the apical system is not alone sufficient ground to separate the 

 Echini into two suborders of equal value. 



We have among the Clypeastroids, as they are here understood, structural 

 features and combinations such as are not known in any other group of 

 Echini, and which prevent us from associating with either of the usually 

 recognized suborder- those sea-urchins in which we find the upper and lower 

 floors, the actinal and abactinal surfaces, connected by limestone walls, pil- 

 lars, or radiating partitions. The suborder is strictly intermediate between 

 the Desmosticha and Petalosticha, having the petaloid structure of the am- 

 bulacra and anal opening disconnected from the apical system, giving US, 

 as in the latter, the position of an anterior and posterior extremity, and the 

 jaws of the former, though simpler and greatly modified, and moved by a 

 totally different mechanism, articulating upon the auricles instead of being- 

 held in place by a muscular system as in the regular Echini. They are \/- 

 shaped, placed horizontally, and in a grc< ve corresponding to the line of 

 junction of the two ossicula are placed the teeth. The structure of the 

 spines is more similar to that of the regular Echini. The ambulacra! pores 

 of the petals pass between the plates : the ambulacra] system is greatly 

 developed, lines of minute pores extend at right angles to the general 

 course of the poriferous zone. On the actinal surface there are ambu- 

 lacra] furrows crowded with pores arranged without any regular order ; 

 the furrows terminate at the actinostome in the buccal tubes ; in some of 

 the families the plates round the actinostome are cuneiform, forming a 

 buccal rosette. The ambulacra are broader than the interambulacra ; in 

 many of the Laganoids the interambulacra are reduced to a narrow band. 

 Tubercles small, perforate, and crenulate. 



