count the number of monospondylous vertebrae 

 with confidence, only these counts are given here 

 (table 1). 



The series reported here is too small for defini- 

 tive use, but is sufficient to give promise that a 

 more comprehensive study would be helpful in 

 species diagnosis. Several kinds of interpretive 

 factors may require consideration in a more com- 

 prehensive study. Some variation in meristic 

 characters would not be surprising in a sample col- 

 lected over a large geographical range. In our 

 series of 33 counts from Apristv/rus indicus, the 

 counts vary from 35 to 41, and localities of capture 

 range over 20 degrees in latitude. In the series of 

 25 counts for Aprixtvrux Imiruxxoni a single count, 

 higher than the cluster, is the count for the holo- 

 type, taken about 20 degrees north of any of the 

 other specimens of the series. 



The reliability of counts of vertebrae for pur- 

 poses of numerical analysis or for identification is 

 reduced because of abnormalities occasionally 

 present. In the distribution of counts in 34 speci- 

 mens of Schroederiohthys maculatus, for example, 

 where 33 are clustered but one count is somewhat 

 apart, the high count in this specimen appears to 

 be attributable to an uneven transition to diplo- 

 spondyly. Here a few displospondylous verte- 

 brae, appeared well forward in the trunk region. 



MORPHOMETRICS 



The conventional ways to measure sharks are 

 comprehensively illustrated by Garrick and 

 Schultz (1963, figs. 1A and IB), and this system 

 of measurement is used here insofar as possible. 

 Unfortunately measurements very useful for some 

 sharks are difficult to apply to others. In the 

 scyliorhinids, for example, the axis of the caudal 



fin is not perceptibly elevated and the point at 

 which the upper lobe of the caudal fin begins is 

 merely an estimate. The fins of scyliorhinid 

 sharks, especially Apristurus, are soft and are gen- 

 erally rounded or lobelike structures making fin 

 measurements difficult to define. 



Most diagnostic accounts of scyliorhinids rely 

 greatly upon morphometries as key characters to 

 separate species or genera. These characters are 

 at least accessible and their use does not require 

 a microscope. As applied to scyliorhinids, how- 

 ever, some kinds of comparative measurements 

 seem to be particularly unreliable. 



In comparing fin sizes, for example, the state- 

 ment that the length of the base of the anal is 4 

 times the length of the base of the first dorsal fin 

 in one species, but only 2i/> times the length in an- 

 other species, introduces a variety of difficulties in 

 practical application. Precise measurements of 

 the length of fin bases are not easily made because 

 of the gradual slope of the forward end of the fin. 

 This is sometimes complicated by the distortion or 

 shriveling of the specimen in preservation. The 

 lengths of fin bases may vary independently. 

 Thus a comparison that seems quite clearly to dif- 

 ferentiate species when some specimens are com- 

 pared may be quite inadequate for other 

 specimens. 



In scyliorhinids there are occasionally speci- 

 mens having much longer gill slits on one side than 

 on the other, and gill slits are especially subject to ' 

 distortion in length or even in position by 

 preservation. 



The ease with which data on measurement can 

 be communicated and the difficulty of correctly 

 and adequately describing shape seem to have led 

 to overemphasis of measurements and less than 



596 



U.S. FISH AND WILDLIFE SERVICE 



