'i'\:\\\> j.i/Ai;i)s OF 'riii; crnus cnkm ikoi'iioktts 223 



<julaj-/.s .stock alter the ,se[)ar:iti()ii ot" -oiitlu'i-ii J^ower Calil'ornia from 

 the Mexican mainhind. it seems just as h)<»ical to assume that l)()th 

 have descended from a connnon, more northern stock, fes.sef/afu.s 

 representing tlie most progressive element. 



As suggested al)o\"e. perhaps the earliest form to hi'come ditt'eren- 

 tiated from tlie proto-stock was maxiunix, which shows a number of 

 the apj)arently less specialized characters. This form differs from 

 tesselldtiix chielly in its coloration and in its larger maximum size. 

 The retention of a single, wide, irregular-edged, longitudinal stripe 

 on (he lower ])ai't of each side in adults and the appearance of 

 liliiish in the \entral coloration of the young (both modifications 

 are also seen at times in te,sscllati(x) suggest (jnhiris of the scxllneafu.s 

 groiij) from which the ancestral stock of the tcsfiellatus grouji is 

 presumed to have been derived. 



The time of the derivation of max'nnus (se[)aration from the 

 stock that gave rise to the modern tesseJlatus and its derivatives) is 

 ])r()blema(.ical. An extensive submergence of the Lower Californian 

 land mass is thought to have taken place during the post-Pliocene 

 period (see Schmidt, 1922), and this is held to have separated the 

 Cape region from the northern part of the peninsula. Although the 

 last extensi\'e submergence is said to have occurred later, during the 

 post -Pleistocene, jna.i'/nnis was probably isolated before this, that 

 is. in the post -Pliocene. This is suggested by its degree of differen- 

 li.'.lion and its present isolation from the modern tesseUatus. 



\ consideration of the group as a whole, and hence of the proto- 

 typic forms, ///axhnus and tessellafiis, is to be given in the general 

 discussion at the end of this work (pp. 251-2G0). The present con- 

 sideration is therefore limited to the origin and relationships of the 

 seven obvious derivatives from tesseUatus. 



As already indicated, ruhidU'S is a subspecies of tesseUatus., inter- 

 grading with that form in the north, along common mainland 

 bomuhiries and on San Marcos Island. Its range extends south- 

 waid on the jjcninsula and the closer neighboring islands to meet 

 and partly overlap the range of maxlmus. Intergradation with 

 maxhnus is not apparent. Ruhidus diifers from tesseUatus through 

 a significant change in the arrangement of the dorsal markings and 

 in the presence of reddish in the ventral coloration. This slight 

 colorational differentiation of ruhidus from tesseUatus and its ab- 

 sence from the more distant islands, at once suggests a compara- 

 tivel}^ recent derivation, Avhich, in this case, may be assumed to have 

 taken place during the Pleistocene age some time prior to the last 

 extensive submergence of the Lower California mainland. This hast 

 geological change then resulted in the isolation of sections of the 

 ruhidus population on this islands where ruhidus occurs to-day. 



