DEEP-SEA FAUNA. Ill 



Among the deep-sea Echinodermata there are representatives of 

 all the modern orders crinoids, brittle-stars, star-fishes, urchins, 

 and holothurians but none of the ancient palechinoids, cystids, or 

 blastoids are known. The pear-encrinites (Apiocrinidaa), for a long 

 time supposed to have become extinct with the Mesozoic era, con- 

 tinue their succession in the genera Rhizocrinus, Bathycrinus, and 

 Hyocrinus, forms more strictly abyssal in character than the Penta- 

 crinus, whose greatest development seems to be confined to a zone 

 of a few hundred fathoms. Bathycrinus gracilis has been dredged 

 in water of a depth of two thousand four hundred and thirty - five 

 fathoms. 36 



The Asteroidea (star-fishes) and Ophiuroidae (brittle- stars) are 

 diffused throughout all the oceanic zones that have thus far been 

 dredged, the former abounding more particularly at moderate 

 depths. The singularly aberrant, and universally distributed, genus 

 of star-fishes, Brisinga, is one of the commonest and most distinct- 

 ive forms of the abyssal fauna, being found in all depths from four 

 hundred to three thousand fathoms. Of the brittle-stars, of which 

 there are about five hundred species described, more than two hun- 

 dred are restricted in their range to a zone of thirty fathoms. De- 

 spite this apparent localisation of the species to a shallow belt, there 

 are no less than sixty- nine species which descend below one thou- 

 sand fathoms, and about eighteen below two thousand. None of 

 the genera have been positively identified with fossil forms, although 

 not unlikely the Jurassic Ophioderma may in part belong to Ophiura 

 or Pectinura. The affinities of the Triassic Aspidura are still doubt- 

 ful. 3T The relationship existing between the modern echinoid fauna 

 and the faunas of past geological periods is much more marked ; 

 indeed, this relationship may be considered as one of the most dis- 

 tinctive features of the deep-sea fauna. Not only do a considerable 

 number of the living genera date back to the Cretaceous period, 

 but a fair proportion of those of the families CidaridaB, Echinidse, 

 Salenida3, &c., are already found in the deposits of the Jura, the 

 Lias, and even in the Trias. The Tertiary genera are very largely 

 developed, and the utmost similarity prevails, even among the spe- 

 cies. So close is the identity existing between the West Indian 

 urchins and those occurring fossil in many of the European Tertiary 

 beds (older and median Tertiary), that it becomes practically im- 

 possible, or nearly so, to distinguish between the species. 88 This 



