XXXII. CHIROSTOMA 



17 



secured by Dr. Meek at Ocotlan, Jalisco, Mexico, in the Rio Grande de 

 Santiago, June 2 and 3, 1901. 



This species most closely resembles C. grandocule of Lake Patz- 

 cuaro, and C. chapal^ of Lake Chapala. From grandocule (topo- 

 types examined) it differs in the smaller size attained; in the deeper 

 and more compressed body; more translucent flesh, particularly that of 

 the head ; in the longer head, larger eye and sharper snout, and in the 

 larger size of the scales. The proportional differences, which, though 

 evident, are not wide in the adults, are accentuated in the young. From 

 specimens of C. chapala, with which the numerous types were taken, 

 CONSOCIUM differs constantly in the smaller size of the scales, the deeper 

 body, larger head, more pointed snout, and the longer and much less 

 oblique jaws. 



Three specimens in the large series of the two species taken to- 

 gether at La Palma, while agreeing with consocium in the fineness of 

 their scales, have the smaller 'more oblique mouth of chapala, and are 

 definitely intermediate between the two forms in the length of the 

 head and depth of the body. It seems certain that these specimens do not 

 exhibit the extremes of the continuous variation of either species. We 

 interpret them as possibly hybrids between chapala and consocium. 

 The distinctive characters of the two species are contrasted with those 

 of the supposed hybrids in the following table : 



Depth of body 



Length of head 



Length of mandible 



Transverse scale rows 



Longitudinal scale rows 



Scale rows between origins of dorsal 

 fins 



C. CHAPALA 



5.2 to 6.2 

 4.0 to 4.4 

 2.5 to 2.7 

 44 to 52 

 12 to 14 



6 to 9 



Hybrids ? 



5.0 to 5.4 

 Z.7 to 3.85 

 2.6 to 2.7 

 57 to 61 

 16 to 18 



10 to 12 



C. CONSOCIUM 



4.7 to 5.3 



3.2 to 3.6 



2.3 to 2.5 

 53 to dZ 

 17 to 19 



10 to 13 



Body rather strongly compressed and moderately slender ; the dorsal 

 and ventral contours evenly converging, like the edges of a spear-point, 

 to the sharply pointed tips of the snout, which is lower than in chapal^e, 

 being on a horizontal passing through middle of eye and thence through 

 middle of body; greatest depth of body, 5.3 (4.7 to 5.3)^® in length 

 without caudal; least depth of caudal peduncle, 2.0 (1.8 to 2.2) in its 

 length behind end of anal base, a little longer (or a little shorter) than 

 length of snout. Length of head (including opercular membrane), 

 3.3 (3.2 to 3.6) in length of body without caudal. Fore part of head 



26 The measurements and counts in parentheses are those of ten or more 

 paratypes 60 to 74 mm. long, from La Palma. 



