420 SCIENCE PROGRESS 



glossate Cypraea all the teeth of each of the three tracts have 

 been fused into one large tooth so as to give the formula 

 3. I. 3. In the Rachiglossate Nassa fusion has reached the 

 still further extreme of i. i. i. Finally, the central tooth 

 may be absent, as in the Toxoglossate Conus with the formula 

 I. o. I, or as in the Ptenoglossate Janthina and Scalaria with 



X. o. X. 



In spite of the high degree of specialisation in different 

 directions which is so characteristic of Crustaceans at the 

 present day, we find on closer examination that among the 

 various classes there are still in existence some remarkably 

 persistent and even generalised types. One of the most 

 striking instances of such persistence is furnished by the 

 discovery in 1893^ of the Tasmanian Mountain-shrimp {Anas- 

 pidcs tasitiauice, G. M. Thomson), for which a new division — the 

 Syncarida — has had to be created,^ equivalent in importance to 

 the Phyllocarida containing the equally archaic form Nebalia. 

 Anaspides can indeed hardly be distinguished from the Carboni- 

 ferous Praeanaspides discovered by Dr. Moysey^ in the Coal 

 Measures of the Nottingham coalfield in 1907. 



Nebalia similarly represents a remarkable persistence of an 

 extremely ancient and generalised type, with which Hymeno- 

 caris of the Cambrian, and the Ordovician Ceratiocaris ■* and 

 Caryocaris are combined to form the separate class of the 

 Phyllocarida or Leptostraca, intermediate between the 

 Entomostraca and the Malacostraca. The even more gener- 

 alised Apus is known from the Trias and seems to have been 

 represented by the very similar Protocaris of the Lower 

 Cambrian of North America. In point of fact, in the face of 

 all these long survivals from the Palaeozoic era down to the 

 present day, it becomes really a matter for surprise that such 

 successful classes as Trilobites and Eurypterids should have 

 died out altogether when all the other and apparently more 

 specialised classes of Crustaceans and Arachnids of Palaeozoic 

 times should have left descendants existing at the present day. 



' Thomson, Trans.' Linn. Soc. (2), vi. 1894-7, p. 285. 

 " Caiman, Tra?is. Roy. Soc. Edinburgh, xxxviii. 1897, p. 787, 

 ^ Geol. Mag. 1908, p. 385. 



* The Carboniferous Ceratiocaris scorpioidcs and C. elongatus have been, 

 however, considered by some to be Cumacea, but this view seems hardly tenable. 



