90 ECHINOIDEA. I. 



St. 90 (64 45' N. Lat. 29 06' W. L. 568 fathoms. 4 bottom temp. Mud. ). 2 specimens. 

 - 97 (65 28' 27 39' 450 5 i ). 3 



Further three specimens have been taken by Ryder (1888) on 553 fathoms in the Denmark Strait. 



Thus this species also belongs to the rich archibenthal fauna of the northern Atlantic; it is 

 scarcely to be doubted that it is also found in other places than in the Denmark Strait and on the 

 ridge south of Iceland. 



On the Fam. Echinometrads Gray and the Subfam. Triplechinidas A. Agass. 



It has been shown in the preceding, how little successful the previous attempts at a classifica- 

 tion of the Cidarids and Echinothurids have been. It is still worse with regard to the forms that are 

 to be treated here. In the former only the species and genera were confused; here not only the 

 species and genera, but also the families have been mingled to such a degree, that species which have 

 proved by a closer examination to belong to at least three different families, have been referred to 

 the same genus (Strongylocentrotus}. The family Echinometrid and the subfamily Triplechinida 

 prove to be interwoven to such a degree, that it is impossible to treat each group separately. I have 

 examined almost all the genera and species referred to these groups, and have found the relation 

 between these numerous forms that all look rather uniform, to be widely different from what has 

 formerly been supposed although these suppositions have otherwise been sufficiently different. 



The earlier attempts at a classification of the forms belonging here, have been put together 

 by Lilt ken, to whose paper I shall only here refer 1 ). Gray is the first author, who has tried to 

 arrange the genera into families; he establishes the following system 2 ): 



Fam. Hipponoidse. The ambulacral areas as broad as the interambulacral areas; the pores form 

 three separate series. -- Amblypneustes, Boletia, Hipponoe, Holopneustes. 



Fam. Echinidse. The ambulacral areas half as broad as the interambulacral areas; the pores form 

 arcs of 3. A. With pores at the sutures. Mespilia, Microcyphus, Salmacis, Tcmnopleurus. B. With- 

 out pores at the sutures. Echinus, Psammechinus, Heliocidaris. 



Fam. Echinometradse. The ambulacral areas half as broad as the interambulacral areas; the pores 

 in arcs of 4 or more. A. Test round: Strongylocentrotus. B. Test oblong: Echinometra, Holo- 

 centronotus, Colobocentrotus. 

 In the following time repeated attempts have been made to improve the system, but none of 



these attempts have been very successful. A short survey of these systems is given here. 



Troschel (403. p. 297). (No genera are named.) 



Fam. Echinidse. Pores trigemiuate; mouth-slits insignificant; no ocular plate reaches the 



periproct. 



Fam. Tripneustidse. Pores tri geminate, mouth-slits deeper than broad; two ocular plates 

 reach the periproct 



) Bidrag til Kundskab om Echiniderne. Kobenhavn 1864. p. 84 f. (Vid. Medd. Naturh. Foren. Kbhvn. 1863.) 

 2 ) An arrangement of the families of Echinida, with descriptions of some new Genera and species. Proc. Zool. Soc. 

 i855- P- 35-39- 



