258 BULLETIX 15 4, rXITED STATES NATIOISTAL MUSEUM 



absence from the more distant islands both STig'<i;est a comparatively 

 recent derivation, and this ma}' be assumed to haA*e taken place dur- 

 ing the Pleistocene age some time prior to the last extensive sub- 

 mergence of the Lower California region. 



The remaining six forms of the tesseJlatiis group occur exclusively 

 on islands in the Gulf of California and all are clearly recent deriva- 

 tives from tessellatus and imhklus. The two derivatives from riibidus 

 are cendhensis of Ceralbo Island and celeripes of San Jose Island. 

 The four remaining insular forms, catms of Sal Si Puedes and the 

 San Lorenzo Islands, mat'fi/ris of San Pedro Martir Island, hacatus 

 of San Pedro Xolasco Island and catalinerisis of Santa Catalina 

 Island, are all dwarf derivatives from tessellatus^ presenting a com- 

 mon variation, the loss of the longitudinal arrangement in the dorsal 

 markings, which is so characteristically evident in typical tessellatus^ 

 and the development of a tendency to possess a finely reticulated or 

 finely spotted color pattern. 



The hyperiitlirus group is unique in the normal possession of one 

 frontoparietal plate instead of two. It is also characterized by 

 the retention of a striped pattern throughout life and by the un- 

 stable character of the dorsal lines. It has been shown that lahlalis^ 

 the most specialized member of the sexlineatus group, wdiich occurs 

 in northern Lower California in the same general range as hypery- 

 thrus^ the prototype of the hyperythrus group, also retains the striped 

 pattern throughout life and shows a similar variation of the dorsal 

 lines. However, lahialis exhibits a number of modifications which 

 depart as much from normal hyperythrus as from normal perplexus. 

 its ancestor in the sexlineatus group, and these are, of course, of 

 little importance in determining relationships. Otherwise lahialis 

 differs from hyperythriis chiefly in the presence of two frontoparietal 

 plates instead of one. Therefore, it appears that lahialis shows 

 transition from perplexus of the sexlineatus group to hyperythrus 

 of the liyperythrus group, having become specialized somewdiat in 

 the direction of the latter, only to have its characters fixed, probably 

 through the agency of isolation, before the union of the fronto- 

 parietal phites took place. This latter change, probably occurring 

 no earlier than the middle Pleistocene, no doubt marked the birth 

 of a new section of Cneinidophoims^ the liyperytlirus group, this 

 being based at first on a monotype, a species closely resembling, in 

 all probabilities, the present hyperythrus. 



The hyperythi^s group is confined to the Lower Californian re- 

 gion and southern California, and is composed of four forms, the 

 prototype, hypertlirus., and three insular derivatives, caeruleus., dan- 

 heima^^ and pictus, all of which are very closely allied to the parent 

 form. 



