GASTROPODA. d 817 
Olassification. ] 
periphery. Length should be applied only to patelliform shells, in which it refers 
to the distance from the anterior to the posterior margin. A whorl represents 
a single complete revolution of the spiral cone; its periphery is an imaginary spiral 
upon the outer wall corresponding with the line of greatest width. In counting the 
number of whorls we begin with the apertural margin from which to a point 
on the suture next above it constitutes one whorl. Repeating this process to the 
apex gives the total number. The apical angle, which is the angle formed by the 
diverging sides of the conical spire, is determined by means of a goniometer. 
CLASSIFICATION. 
It is very difficult to decide which of the numerous systems of classification 
that have been proposed is the most convenient and at the same time the most. 
natural. The old school of naturalists paid little attention to anything save the 
shell. Another and later school bases a classification almost solely upou the modi- 
fication of the lingual dental apparatus. As neither method has proved entirely 
satisfactory, the most recent authorities are seeking to frame a system that will 
combine the best features of previous classifications. But it cannot be denied 
that the system that will do full justice to the evolution of the class is still a 
thing of the future. Ontogeny and chronogenesis will have much to do with it, 
from which it is obvious that a large proportion of the work must fall to the 
paleontologist. 
The classification adopted by Zittel in his “Handbuch der Palzontologie,”’ 
embraces all the Mollusca which are provided with a tongue in one class, the 
Glossophora. These are divided into four subclasses, the Scaphopoda, Placophora, 
Gastropoda, and Pteropoda. Nicholson retains the first two of these subclasses as 
distinct classes, and unites the last with the third. The Gastropoda he divides into 
two primary groups or subclasses, the Branchiogastropoda and the Pulmogastropoda. 
The Branchiogastropoda again he divides into four orders, the Prosobranchiata, 
- Opisthobranchiata, Pteropoda and Heteropoda. Tryon, in his great work on “Structural 
and Systematic Conchology,” arranges the same organisms as follows: class Ptero- 
poda; class Gastropoda, subclasses Prosobranchiata, Opisthobranchiata and Pulmonifera; 
class Scaphopoda. Fischer’s arrangement again is different: class Pteropoda; class 
Gastropoda; subclass Univalvia, orders Pulmonata, Opisthobranchiata, Nucleobranchiata 
and Prosobranchiata; subclass Multivalvia, order Polyplacophora. 
If we should accept any of the foregoing arrangements it would be Zittel’s, but 
as his scheme is not as well balanced as it might be a few changes are suggested in 
the following brief characterization of the principal divisions. Perhaps a more 
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