ANNELID GENUS CAMBAKINCOLA — HOFFMAN 297 



section) are all specialized both in their respective sections as well as 

 in the entire genus. 



It is, unfortunately, not possible to date the course of events in a 

 postulated phylogenetic scheme in any but the roughest terms. We 

 can assume from paleobotanical evidence a widespread occurrence of 

 the so-called Arcto-Tertiar}^ forest across midland North America as 

 late as the Miocene, and the gradual conversion of this area into 

 semiarid grassland in the Pliocene. This change of basic habitat 

 must have commenced a schism of earlier broad ranges which was 

 culminated by Pleistocene glaciation. 



The apparent lack of endemism in glaciated parts of the continent 

 suggest both a low rate of evolution and relative recency of post- 

 glacial northward migration by crayfish. The presence of C. jphila- 

 delphica in Wisconsin is in all probability due to its isolation there in 

 the well-known driftless area. At present I know of no authentic 

 records for the species between Wisconsin and New York. 



A very provisional reconstruction of the events by which Camha- 

 rincola has undergone dispersal and evolution in North America might 

 be about as follows: 



1. The procambaruicolid stock arrived on this continent from 

 northeastern Asia, as commensals on primitive astacine crayfish 

 perhaps during late Cretaceous or early Tertiary times. During 

 this period western North America was subjected to considerable 

 uplift and down warping ; large extensions of the sea covered much of 

 the southwest and it seems doubtful that crayfish were able to occupy 

 much territory until the Eocene brought widespread emergence and 

 generally uniform subtropical climate to the continent. The spread 

 of the cambarine cra3^fish (which presumably evolved in America by 

 the loss of branchial elements and specialization of certain pleopods) 

 into eastern North America could scarcely have taken place prior to 

 withdrawal of the midcontinental Cretaceous embayments. 



2. The species of the Mesochorea section of the genus developed 

 in something Uke their present form, with numerous species or widely 

 ranging ones (perhaps both) similar to the Recent C. ouachita, occur- 

 ring nearly across the continent. 



3. Perhaps along with the crayfish genera Orconectes and Camharus, 

 cambarincolids occupied the Appalachian system, which was then 

 being developed b}'- dissection of the old Cretaceous peneplain and 

 doubtless afforded a new type of habitat. Here both crustaceans and 

 annelids prospered, and the Philadelphica section had its origin by 

 gradual differentiation of the prostate gland and diminution of the 

 deferent lobes of the spermiducal gland. New and more specialized 

 forms continued to evolve here, doubtlessly enhanced by the oppor- 

 tunities for locaUzed geographic speciation in mountainous terrain, 



