Cvi PROCEEDINGS OF THE GEOLOGICAL SOClETr. [May 1 896, 



The existence of this Astacomorphous type from Palaeozoic times 

 exactly corresponds with its remarkable and world-wide distribution 

 at the present day. The late Prof. Huxley suggested that the 

 descendants of Eryma in Jurassic times gave rise to Enoploclytia 

 and Hoploparia in the Cretaceous period, and these to the modern 

 marine Homarina : per contra — that Pseudastacus, in the Jurassic, 

 originated the freshwater Potamobiidae, which have, in the long 

 period of time that has since elapsed, not only split up into the 

 northern and southern potamobine and parastacine types, but have 

 become distributed from land to land by overland and freshwater 

 lines of communication, since broken up and removed, but which 

 must, upon this hypothesis of descent, have formerly existed ; unless 

 we are prepared to adopt the theory of geographical distribution of 

 animals, propounded by St. Augustine for insular floras and faunas 

 — namely, that they were carried there by angels. 



The magnitude of the problem of the Astacidea becomes more 

 apparent when we bear in mind that these freshwater Crayfishes 

 are distributed over the rivers and lakes of 12 widely separated and 

 extensive land-areas (each being marked by its own geographical 

 species), such as the European-Asiatic area, the Amurland, the 

 Japanese, the West North American and the East North American, 

 the Brazilian, the Chilian, the New Zealand, the Pijian, the Tas- 

 manian, the Australian, the Madagascar areas. 



It is not without interest to observe the strong cousinly resem- 

 blance between many of these forms now separated so far by time 

 and space. Take, for instance, two of the largest living forms — the 

 Astacoides madagascariensis, from Madagascar, and the Astacoides 

 armatus, from the Murray Eiver, South Australia ; they closely 

 resemble one another in form and in general structure, although 

 now separated by the breadth of the great Indian Ocean ; but the 

 former species is smooth, or only slightly scabrous, while the latter, 

 as its name suggests, is armed with prickly spines on the sides of 

 the cephalothorax and the abdominal somites. 



This spinose ornamentation may seem trivial, but it is, I believe, 

 unique of its kind among living Astacides. Strange to say, it finds 

 its homologue not in another living or fossil form of Crayfish, but 

 in an ancestor of the marine branch of the Astacidea, the Enoplo- 

 clytia sussexiensis from the Chalk of England. 



The Thalassinidea form a remarkable family of true fossorial 

 Macrura, having a long and slender abdomen, the segments of which 

 do not overlap, and the epimera are feebly developed ; , they have also 



