•200 THE VOYAGE OF H.M.S. CHALLENGER. 



" The absence of the Clypeastroids from the deeper waters is interesting, indicating that 

 they probably developed rapidly during the Tertiary period, and have always been (as they 

 are to-day) inhabitants only of shallow seas. 



" By far the most interesting Echini collected by the Challenger belong to the strictly 

 deep-sea types, the Pourtalesise and Ananchytidse, families of which the nearest allies were 

 known only as fossils before the days of deep-sea dredging. The first family, Pourtalesiae, 

 was discovered by the late Count Pourtales.in the trough of the Gulf Stream, between 

 Key West and Havana. The Challenger has added no less than twelve new species to 

 this family. Some of the genera are of the most extraordinary shape, and, like the original 

 Pourtalesiae, seem to have little in common with the normal Spatangoids as we know 

 them from their living and fossil representatives. The slipper-shaped EcMnocre/pis, and 

 the Galerites-like Urechinus remind us of types which flourished in the Cretaceous Seas. 

 One of the species of Cystechinus, with its thin flexible test, looks in alcohol more like a 



Fig. 85. — Powrtalesia ceraliipyqa, A. Ag. Seen from the abactinal side, 

 covered witli spines ; natural size. 



diminutive battered felt hat than the graceful sea-urchin it must have been judging from 

 its hard-tested congener. 



" No less than five new species of Ananchytidse were brought home, a family once 

 numerous in the time of the Chalk, and remarkable, like the Pourtalesiae, for their 

 imperfectly developed and simple ambulacra, and for the uniform size of the plates 

 composing the ambulacral and intcrambulacral areas of the test. These two families are 

 also noted for the absence or slight development of the fascioles, so characteristic of 

 nearly all recent Spatangoids, but absent in many of the more recent fossil types and in 

 all the other forms of extinct Spatangoids. 



" Interesting from an embryological point of view are such novel and strange forms as 

 Aerope and Aceste, which have assumed a facies absolutely identical with that passed 

 through by the young of the Brissina of to-day. In these two genera the odd anterior 



