MUSEUM OF COMPARATIVE ZOOLOGY. 87 



fathoms) to the depths of the abyssal region (250 to 2,000 fathoms) un- 

 limited by temperatures actually encountered. 



II. Of the species with great vertical range (from less than 100 to 

 more than 500 fathoms), the smallest part (ten per cent) are of groups 

 which have been regarded as belonging to or characteristic of the shores 

 of cold or boreal areas. The next larger part (twenty per cent) belong 

 to. groups hitherto considered characteristic of shoaler warm or tropical 

 waters, while more than sixty per cent belong to groups not especially 

 characteristic of the litorale of either region. 



III. Of the species found in the abyssal fauna without regard to 

 their vertical range above it, ten per cent may be termed boreal, thir- 

 teen per cent tropical, and more than seventy-five per cent uncharacter- 

 istic forms, 



IV. Since the tropical forms belong to the same groups as those char- 

 acteristic of the local litoral mollusk fauna, it is eminently probable that 

 the abyssal regions have local faunae proper to their various portions, and 

 that a universal exclusive abyssal fauna, so far as mollusks are concerned, 

 does not exist. This must be qualified by the admission of the exist- 

 ence in the abysses (as well as on the litorale) of ubiquitous species- 

 forms ; which, however, do not form a universal abyssal fauna, any more 

 than Mytilus edulis, Saxicava rugosa, and Poronia rubra form a universal 

 litoral fauna. The local nature of different portions of the abyssal fauna 

 is also confirmed by the distinctness of the Challenger mollusks from 

 those of the Blake, but a very small number appearing identical as far 

 as a cursory examination could determine. 



There can be no doubt that the uniformity of generally low tempera- 

 tures (and consequently of food) affords special facilities for the wide dis- 

 tribution of boreal forms through the abyssal region. But where adjacent 

 shores can (by washing and sinking) afford a different or greater variety 

 of food without too excessive temperatures, local abyssal faunae will prob- 

 ably always be developed, and with characteristics assimilated to those 

 of the litoral fauna of the same part of the earth's surface. The present 

 collection shows conclusively that a difference in pressure of some 120 

 atmospheres and in temperature of 41.5 degrees has been sustained by 

 different individuals of the same species without perceptible change in 

 the external appearance of their hard parts or shells. 



V. The specific characters of many of the strictly abyssal species ap- 

 pear to exhibit a very remarkable degree of variation within supposed 

 specific limits, although it would seem as if the conditions under which 

 they live must be remarkably uniform. This would indicate that the 



