1909.] R. B. lyivOYD : Deep-Sea Fish caught by the ''Investigator." 173 



arid broad. If some hundreds of specimens had been available for measurement, 

 there is no doubt that these three groups would have been bridged by intermediate 

 forms, but they are so distinct that it seems certain that specimens approximating 

 to the three types would be in excess however many specimens were measured. 



» The Dermal Ossicles. 



Owing to the smallness in numbers some doubt may possibly be felt as to 

 whether the above threefold division is justified. An examination of the dermal 

 ossicles, however, shows a result which may be stated without doubt. As regards 

 the arrangement of these ossicles the twenty-one specimens fall into two groups, 

 which are perfectly distinct from one another. There are two distinct types in the 

 arrangement of the ossicles, which will be spoken of as the ''type of order" and 

 the ''type of disorder." 



Each specimen possesses three particular spines, — a median nasal spine and two 

 subopercular spines, one at each posterior angle of the disk. It was first noticed 

 that whereas the nasal spine did not vary much in size or form among the whole 

 group, the subopercular spines varied to a remarkable extent. The nasal spine is 

 simple and conical, it was measured in each case by placing one end of the callipers 

 in the little tentacular pit which undermines the spine, the other on the tip of the 

 spine. The measurement so obtained was compared with the diameter of the eye. 

 It was found that in every case the length of the spine was about half that diameter. 

 Though constant in length this spine varies considerably in its position, being nearly 

 upright in some while in others it is directed forwards. 



The subopercular spines, however, vary to an extraordinary extent in size and 

 form. They were studied in the following way: — The right spine of each specimen 

 was placed beneath the microscope; magnified 50 times and its outline drawn with 

 the camera lucida. The resulting figures reduced in size are shown on plate 1. It 

 was found that these spines were in one of two conditions ; they were either small, 

 relative to the size of the fish, and covered irregularly with short projections, or they 

 were large and armed with four long transparent spinelets disposed in a particular 

 order constantly occupying the positions indicated by the terms antero-superior, 

 antero-inferior, postero-superior, postero-inferior. The antero-superior spinelet is 

 much the largest and is directed forwards ; it is marked i in the figures. The 

 postero-superior spinelet is generally curved upwards like a cow's horn; it is 

 marked 2 in the figures, which do not show the curvature satisfactorily. Thirteen of 

 the specimens in the collection possess these relatively large tetrafid subopercular 

 spines, while the other eight have relatively small and irregular spines. 



It has been mentioned that as regards the dermal ossicles, there are two distinct 

 types in the collection, the orderly and the disorderly type. The presence of a large 

 tetrafid subopercular spine is an important part of the orderly character. In 

 every specimen possessing a small irregular subopercular spine the dermal ossicles of 

 both dorsal and ventral surfaces are distributed in a disorderly manner. In every 



