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ALLAN HANCOCK PACIFIC EXPEDITIONS 



VOL. 18 



The present species would be of great value in studying post-ephebic 

 differentiation and ecological influences on size and maturation. The 

 distinct differences between sizes of spines on the first two uropods would 

 be an interesting problem for the genetical ecologist. 



TABLE 5 



Specimens from San Quintin Bay 

 Some evidence for the influence of environment on the expression of 

 morphology is seen in the specimens collected at San Quintin Bay, Lower 

 California (Plate 35, figs. S-Y). The adult animals ranged in length 

 from 5 to 6 mm. The lower posterior corners of the third epimera are 

 quite prolonged and upturned, exaggerating the extreme found in larger 

 adults of the open sea, but the second article of the fifth peraeopod is not 

 broadly expanded and the teeth are quite distinct. 



Juvenile animals 

 The juveniles of this species are quite similar to adults of P. spinosus 

 and difficult to distinguish, for the head is not prolonged and the ar- 

 mature of the first two uropods is quite similar in both species. It is 

 interesting to note that the apical spine of the fourth maxillipedal palp 

 article, seen in the adults of other species, is present in juveniles of P. 

 obtusidens, although it is rather indistinct. Because of its loss or fusion 

 in maturation it would appear that species which retain it as adults are 

 more primitive. 



