66 BULLETIN OF THE 



and specific peculiarities long before the characteristics of the outer skin 

 are acquired, even before the shapes of body and fins, and the peculiar 

 dentition, or the other features commonly relied upon to separate the 

 forms and groups, have become available. Consequently, the various 

 embryos may be recognized by means of the canals at periods when 

 identification by the specific and generic characters ordinarily em- 

 ployed for the purpose of distinguishing them would be quite out of the 

 question. 



Dichotomized tubules do not appear to any great extent on the lower 

 surfaces of such as habitually lie on the bottom. The tubules which 

 occur on the ventral canals of such species most often have their ex- 

 ternal apertures at the border of the disk. On the Torpedinida? it is 

 altogether likely that disuse has led to the loss of the entire ventral 

 portion of the system. Types addicted to flights through the water at 

 a distance from the bottom have the tubules and their branchlets more 

 alike on the upper and lower surfaces, as may be seen in such as the 

 Zygobatidse and the Dicerobatidae. These and similar Rays have a 

 greater aggregation of the tubules and branchlets toward the hinder 

 portion of the disk, a distribution which suggests liability to danger 

 from behind, possibly while the creatures are feeding. The modifica- 

 tions in position and outline which the cephalic canals have under- 

 gone on these families, through changes in shape of head, snout, and 

 pectoral fins, and through change in the position of the mouth, become 

 very prominent when directly contrasted with the same canals on such 

 as the Trygonidae or the Raiidee. 



The extent to which the canals may be used in classification is best 

 illustrated by comparing the systems on the various species or groups. 

 Necessarily the comparisons instituted below have been made very 

 much as if the genera were composed of the species under examination. 

 Further investigation of other species will, no doubt, bring to light dif- 

 ferences in matter of detail, serviceable in specific diagnoses, and possibly 

 such as may compel modification of our ideas of the generic charac- 

 ters ; but the attempt has been made here to use only such features as 

 are least liable to the minor variations. As the vessels have not yet 

 been studied on one sixth of the whole number of species in the class, 

 and as those on which the system has been worked out do not include 

 representatives of all of the genera, it follows that a synopsis con- 

 structed on the material here gathered could only be a temporary affair. 

 For this reason a short summary of differences is to be preferred to a 

 synopsis giving only a few of the more prominent ones. 



