198 



ME, C. TATE REGAN ON THE 





i. 



2. 



3. 



4. 



5. 



6. 



7. 



8. 



Gen. XVI. Arges (con.). 



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12. A longifilis Stdr 



13 A sabalo C Sf V: 













Gen. XVII. Astroblepus Humboldt. 





Before passing to the systematic portion of this work, it may be as well to 

 summarise the more important sexual differences and the changes which take place 



during growth. 



Sexual Differences. 



The differences between the sexes in certain genera are very remarkable. In the 

 forms with the body protected by bony scutes it is often the case that the little spines 

 which cover these and the plates of the head are stronger in the males than in the 

 females. This feature is greatly exaggerated in Plecostomus spinosissimus and P. festce, 

 in which the males have the head and body covered with quite strong spines. In 

 other cases specialisation has resulted in the great development of spines or bristles in 

 certain areas only. Thus in Ancistrus setosus and its allies, and in many species of 

 Oxyloricaria, Farlowella, and Loricaria, the sides of the head in the male are margined 

 with bristles, which are either absent or considerably shorter in the female. 



In Loricaria lanceolata and its allies similar bristles occur in addition on the supra- 

 occipital, the nuchal scutes, and the spine of the pectoral fin. In many species of 

 Loricaria which have the sides of the head bristly in the males, the head is also some- 

 what broader and the snout more obtuse in specimens of that sex, a feature which 

 is perhaps more strongly marked in L. latirostris than in any other species (see 

 PL XVI.). 



In most species of Xenocara the naked margin of the snout is much wider in males 

 than in females, and whilst the latter have a more or less distinct marginal series of 



