388 ORTMAXX — DISTRIBUTIOX OF DECAPODS [Aprils, 



extended over a great part of Mesozonia, from southeastern Asia 

 over northwestern America to southern Mexico, and, in more 

 primitive forms (approaching Astacoides), extending even from 

 southern Asia across India to Madagascar. 1 



3. In Lower Tertiary times, the genus Potamobius gives origin, 

 in Mexico, to the genus Cambarus, and this spreads over the 

 eastern parts of North America. The Parastacidce of Archinotis 

 extend from Chili to northern Argentina and southern Brazil, and 

 the family is divided into an Australian group (which splits up in 

 several genera) and a group belonging to Neonotis (Parastacus). 



4. In Upper Tertiary times (and later), the Potamobiidce from 

 eastern Asia reach western Asia and Europe, and the Parastacidce 

 become restricted to Australia, New Zealand and South America. 2 



C. History of the Distribution of Freshwater Crabs. 

 (Compare Fig. 3, p. 297.) 



1. In the Upper Cretaceous, freshwater crabs o<" the family 

 Potamonidce existed in parts of Mesozonia, beginning in India 

 (possibly going eastward to the Malaysian islands), and extending 

 over the Lemurian bridge to Africa, Guiana and Central America. 3 



2. In the beginning of the Lower Tertiary, we find this area 

 divided into two main portions. The one comprises parts of 

 America : the northern parts of Neonotis and the southern parts of 

 Sino-Nearctica (northern Central America), which are again sepa- 

 rated from each other. These regions are inhabited by the sub- 

 family Potamocarcinince. The second main portion, occupied by 

 the subfamily Potamonince, is formed by Africa and India, and 

 this, during this time, is again divided into two sections, an 

 African (including Madagascar) and an Indian. 



3. In the Upper Tertiary (and later), the two sections of the range 

 of the Potamocarcinince become reunited, so that this subfamily now 

 occupies the West Indian region, Central America and northern 

 South America. Also the immigration in the Lesser Antilles 



1 The possible cause of the check to the farther distribution of the crayfishes 

 over Mesozonia, westward beyond Madagascar and south- and eastward beyond 

 Central America, will be discussed below (see p. 391). 



2 The Madagascar! (orm, Astacoides, therefore does not belong to this stock, 

 but should form a group by itself. 



3 It will be noticed that the distribution of crayfishes and crabs in Mesozonia is 

 almost mutually exclusive : they came into contact only in Lemuria (and South 

 Asia) and northern Central America. See below, p. 391. 



