The Genus Gyrotoma 7 



som constitute a third group whose relations with Goniohasis impressa are 

 quite plain. Incisnm, zvalkeri and amplmUy though not always easily to be 

 distinguished from one another, manifestly differ from the other species of 

 Gyrotoma. The fifth group is made up of excisum and lacimatuni. Their 

 well-fixed characters and their long range in the river justify the belief 

 that they are the oldest species of the genus. I have been unable to find in 

 Goniobasis any species definitely ancestral to the excisum and incisum 

 groups. 



The specific value of some of the races of Gyrotoma, I recognize, is open 

 to question. It requires considerable familiarity with them, and perhaps 

 the habit of mind which goes with studying a family of extreme complexity, 

 to be altogether confident as to the identifications of certain forms, incisnm 

 and ampltim for example. Characters overlap. Characters sometimes 

 prominent in one locality fade nearly to invisibility in another. It might 

 possibly be doubted whether variations in the depth of the fissure, being 

 largely a matter of size, warrant recognition beyond subspecific rank. 



But difficulties arise in attempting a wholesale lumping of the Gyroto- 

 mae. One would have to ignore distinct differences in many of the young 

 and these differences point to ancestral distinctions. The derivation of one 

 group is clearly not the derivation of two other groups. A fourth and a fifth 

 group cannot be traced to any known form. Lumping would consolidate 

 forms independently developed, adding confusion instead of achieving sim- 

 plicity. 



A possibly important question in connection with Gyrotoma is whether 

 it is a genus in course of developing or disappearing. J. C. Willis (11, p. 

 166) recites three explanations for endemism which he chooses to consider 

 rivals: "(i) That endemics are very specialized species (and genera) suited 

 only to the areas upon which they have been found; (2) that they are old 

 species (and genera) which have been driven into quiet nooks or left in odd 

 corners by the competition of better adapted species; and (3) * * * that 

 in general they are young beginners, descended from the 'wides'." Willis' 

 argument is largely devoted to the advocacy of the third explanation as the 

 one for the majority of cases. I see no reason why all three hypotheses may 

 not apply with almost equal force. The Vitrinizonites of the southern Ap- 

 palachians may be examples of highly specialized forms narrowly conlined 

 to particular localities. An illustration of one foini of a relict race is given 

 by Ortmann (7, p. 3-6). Marqaritana marqanllfera (L.) occurs in the 

 headwaters of the Schuylkill River, and in no other place in Pennsylvania. 

 The shells are confined to cold trout streams and in the parts of these 

 that are ono to 1200 -^eet above sea level. "The Pennsylvanian area of 

 this spec-f's is not onlv the most southern extension of its range in eastern 

 North A'nerica. but it also has the peculiarity of being the only one to the 

 south of the Terminal Moraine. Thus it may be regarded as a part of the 

 Glacial Preserve (refugium) of this species." The great numbers in which 

 Gyrctoniae occur in the Coosa River, their variability which is so character- 

 istic of a genus that has not yet become "fixed" and their failure, for all their 

 numbers and apparent vigor, to get beyond a restricted habitat, convince 

 me that they represent "young beginners descended from the 'wides'." 



