252 CALIFORNIA ACADEMY OF SCIENCES. 



parent form, called microcephalics, as compared with the 

 average representative of the derived form, zvilliamsoni^ 

 has: 



1. The sides with more plates. 



2. The pre-pectoral area larger. 



3. The pubic plate longer and sharper. 



4. The caudal peduncle or tail more slender. 



5. The dorsal and ventral spines longer. 



6. The pelvic girdle more firmly attached to the in- 

 terclavicles. 



7. The post-pectoral plate heavier. 



8. The sides of the thoracic area less convex. 



9. The first dorsal plates larger and more firmly at- 

 tached. 



10. A larger average size of the adult individual. 



The fin rays are: zvilliamsoni, dorsal 9 to 12, anal 7 or 

 8; microcephalics, dorsal 10 to 13, anal 7 to 9. 



These numerous differences, together with the differ- 

 ence in distribution, necessitate the separation of micro- 

 cephalics and zvilliamsoni, but the fact that the differences 

 all intergrade in intermediate territory, gives these forms 

 only subspecific value. As the name zvilliamsoni is the 

 older, it must be used as the specific name, while the 

 common or parent form must be recorded as Gasterostcics 

 zvilliamsoni microcephalics. 



The true zvilliamsoni, with the exception of the eight 

 type specimens, is known only from the Santa Ana Basin 

 at Colton, Riverside and San Bernardino. The form 

 called microcephalics is known from many streams from 

 Unalaska on the north to the Santa Clara River on the 

 south, where it meets zvilliamsoni, the types of zvilliamsoni 

 being from a tributary of the Santa Clara. 



The variety microcephalics shows great variations, but 

 I have been unable to coordinate them with any locality 



