i895. RESULTS OF ''CHALLENGER'' EXPEDITION. 45 



some changes in the classification of the urchins. Though Agassiz 

 retains the Palechinoidea as a sub-order, he points out that it can no 

 longer be contrasted with all other types of Echinoidea. Carpenter 

 in his reports, it will be remembered, emphasised the distinction 

 between Palaeocrinoids and Neocrinoids ; and in that case it was 

 reserved for palaeontologists to show that such a division could not 

 be maintained. 



Most important among deep-sea urchins is the elongate Ponvtaksia 

 (PI. ix., Fig. 3), first found by Count Pourtales between Key West and 

 Havana, and the subject of Loven's classical study. To the family con- 

 taining this and other strange shapes, twelve new species were added 

 by the " Challenger." " It was" says Dr. J. W. Gregory, " the close 

 resemblance of some of these to the Cretaceous Ananchytida^ that led 

 to the well-known and oft refuted generalisation that we are still 

 livingin theageof the43halk." Among these the genus Cystechinus with 

 its thin flexible test (PI. ix.. Fig. 2) is of special interest, as presenting 

 in the structure of its plates a surprising similarity to such Palaeozoic 

 fossils as Palceechimis : this genus was not a known fossil till Gregory 

 described Cystechinus crassus from the Pliocene bed of Barbados. 

 Here also are placed the slipper-shaped Echinocrepis, and the Galerites- 

 like Urechinus. These and other deep-sea Spatangoids resemble the 

 fossil, but differ from nearly all other recent Spatangoids, in the 

 absence or slight development of the fascioles. " Interesting from an 

 embryological point of view," says Agassiz, " are such novel and 

 strange forms as Aeyope (PL ix.. Fig. i) 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 ambu- 

 lacrum is immensely developed, its suckers are of a gigantic size, 

 entirely out of proportion to the rudimentary ones of the paired 

 ambulacra." 



It was the dredgers on the " Porcupine " who were the first to be 

 startled by the worm-like movements of the living Echinothuridae, 

 urchins whose flexible tests with imbricating plates were already 

 known as Chalk fossils. To our knowledge of this family much was 

 added by the " Challenger." The young of the group and such 

 species as Asthenosoma gracile, Phonnosoma asterias, and P. ngiduiii show 

 that the lapping of the plates and the distinction between the actinal 

 and abactinal surfaces are features of gradual development, and 

 render it difficult to separate this family from the Diadematidae. 

 Some species of the family present peculiar modifications of the 

 spines. Sometimes baggy envelopes with an irritating fluid surround 

 the sharp spines, and add a numbing effect to their painful wounds. 

 Sometimes the spines end in knobs or in broad conical shoes, as in 

 Phovmosoma hoplacantha ; these serve as pattens to raise the animal 

 from the ground. The Arbaciadae have a similar cap on their spines, 

 and in Coelopleunis it was shown by " Challenger " specimens to be 

 developed to four or five times the length of the spine proper. It is 



