286 Prof. S. Lov(in on the Structure of the Echinoidea. 



side turned towards the median suture of the ambulacra, de- 

 creasing thence the further from the mouth, especially on the 

 bivium, — not unfrequently four, three, or two upon each of the 

 first plates, only one upon each of the immediately following 

 ordinary plates, in Plagionotus, Brissus, SchizasteVy and Mcera 

 {Oualteria?) ^ more numerous on the bivium, in depressions, or 

 like rows of beads in narrow, elongated, well-defined furrows ; 

 but in Lovenia the segregated sphasridia are concealed under 

 domes, which have a small, narrow, transverse opening at 

 their apex. 



A covering of this kind, which is an exception among the 

 Spantangidge, is the rule in the Cassidulid^e and Clypeastrida3. 

 lihyncholampas carihcearum (Lamk.), Pygorrhynchus imcificus 

 (Agass.), and many others have on every plate of the first five 

 pairs in each ambulacrum a sphajridium, which is gradually 

 overgrown by the outer layer of the shell-substance, which 

 finally leaves only a fine fissure open. 



The Clypeastridaj exhibit two types. Echinarachnius^ Den- 

 draster, Lobojyhoraj IfelKta, EiicopCj Rotula, Lagamim^ Scaph- 

 echtnus, and Echinocyamus have in each radius only a single 

 sphgericlium in common for both its pcristomial plates, and 

 most frequently, even in very young individuals, concealed in 

 a crypt in the mass of the shell. Near the peristomial margin, 

 which in the middle has a part somewhat projecting over the 

 two large pores of the buccal tentacles, we see, behind this, a 

 small, more or less distinctly halved elevation. On breaking 

 this up we find a sphteridium with its pedicel attached to the 

 inner surface of a rounded cavity towards the mouth, which 

 is connected with the exterior either only by a fine canal, or, 

 in Rotula^ by means of a tolerably wide opening, which is in 

 part covered by points projecting from its margin. In Echin- 

 arachnius this cavity is divided into two halves by a very 

 thin, vertical membrane, which seems to issue from the edges 

 of the plate united in the suture. It is otherwise with Cly~ 

 peaster and Arachnoides ; these have two sphasridia in each 

 ambulacrum, one in each of its two peristomial plates. In 

 both, the margins are destitute of the projecting part, and the 

 two pores of the large tentacles arc exposed, not, as in the 

 preceding, in a surface falling abruptly towards the mouth, 

 but in a more level and open surface Avhicli nowhere exhibits 

 a sign of the presence of the sphasridium. In Chjpeaster we 

 can only perceive that, at a distance from the tentacular pores 

 twice that of the latter from the margin, the large radial tu- 

 bercles have between them a greater space than elsewhere, but 

 not differing in the disposition of the small tubercles and pores. 

 If we break through the outermost layer of the shell; we find 



