366 THE AMERICAN NATURALIST: | (Vor. XXXIX. 
see for himself something of the main course in the evolution 
of the structure of higher vertebrates. The study of types situ- 
ated near the main line of vertebrate descent is to be preferred 
to the study of extremes of specialization. It seems to me to 
be questionable wisdom to let enthusiasm for local fauna lead 
to the selection of a teleost, for class study, provided that marine 
material is obtainable, and provided further that only one repre- 
sentative of Pisces can be studied. 
Among the elasmobranchs, the sharks depart less conspicu- 
ously from primitive conditions than the rays and therefore, 
theoretically, the dogfishes are to be preferred to the skates for 
general laboratory purposes. The dorso-ventral flattening of 
the skate, however, gives it a certain superiority as a subject 
for dissection. As a result of the flattening, most of the organs 
of the animal are much more easily accessible than they are in 
the dogfish. This advantage is of particular importance when 
it comes to the injection and dissection of the blood-vessels. 
The abdominal cavity of the skate is completely exposed with 
a minimum of cutting. This cavity is a broad and very shallow 
one so that, its ventral wall having been removed, a slight dis- 
placing of viscera brings all its organs immediately into view 
and renders its chief blood-vessels very easy of access. 
It is a consideration of no small moment that the skate, dur- 
ing injecting and dissecting operations, lies in the position most 
favorable for work — that is, flat on its back — without any 
special device for holding it there, while it is not always easy 
to support a dogfish in a desired position. 
In the dissecting and study of the circulatory organs, the 
flattening of the skate is especially advantageous, because, one 
might say, it tends toward the projecting of the blood-vessels 
into one plane, resulting in an almost diagrammatic arrangement 
of them. 
A practical point which will often lead to the selection of the 
skate, if elasmobranch material is to be used, is the fact that 
skates can be obtained during a large part of the year when 
dogfish can not. Mr. F. T. Lane, of Rockport, Massachusetts, 
in response to my inquiry, writes me that he can catch skates 
off Cape Ann at any time of the year, except during storms, 
