65 



probably community exists in the area circumscribed by the 13 stations 

 in fig. 19. 



This same environment probably continues uninterrupted along the 

 coast to at least Panama, but samples were taken at these depths in 

 sufficient numbers only south of Los Mochis to San Bias. A similar assem- 

 blage was also devised in which about one third of the species was taken 

 in common to the two sections. There were, however, a number of inverte- 

 brate species taken in the southern intermediate shelf region which were 

 not taken in the north, and these served to separate the areas by means 

 of the contingency matrix. As with the nearshore shelf, temperature seems 

 to play a part in creating different faunas in the north and south, since 

 many genera have species only found in the north and others only in the 

 south. The greatest difference between the two assemblages may, however, 

 be a result of the difference in sampling. Certainly the large trawls were 

 able to capture more of the large motile decapods and large prosobranchs. 

 These two groups were twice as abundant in the southern region where 

 trawls were used, than in the Tiburon region where they were only used 

 four times. The species most frequently occurring at the seven stations 

 which proved by preliminary computor analysis to be distinct are listed 

 in Table VI. The seven typical stations were taken on silty clay and ranged 

 in depths from 36 to 75 meters, as shown on fig. 22. In order to substantiate 

 the importance of the animals in Table VI for this environment, the same 

 system of computed indeces of uniqueness was computed as was for 

 Table V. It can be seen immediately that no infaunal species (unless you 

 consider the prosobranchs as living in the bottom) figured in the computor 

 analysis. What is significant, however, is that the species in Table VI are 

 very restricted in their distribution, even though they are capable of much 

 greater mobility than the other invertebrates. 



A number of other stations were also occupied in the southern portion 

 of the intermediate shelf. Most of these stations contained the animals 

 listed in Table VI, but not in such a close association. The seven matrix- 

 associated stations constitute a nucleus or center for the whole assemblage, 

 while the others are on the periphery, and have a more heterogenous group 

 of animals. These 12 stations are shown on fig. 22. A list of the living 

 species collected from all 12 of these southern stations is given in Table II. 

 The most abundant infaunal species on sand bottom off San Bias were: 

 the prosobranchs, Calliostoma bonita, Astele rema, Architectonica placen- 

 talis, Natica broderipiana, Natica colima, Polinices uber, Distorsio decussa- 

 tus, Bursa nana, Murex recurvirostris, Hexaplex brassica, Coralliophila 

 hindsii, Cantharus capitaneus, Mitra erythrogramma, Knefastia walkeri, 



5 Vidensk. Medd. fra Dansk naturh. Foren. Bd. 126. 



