OASTU"! >l>\ 



periphery. Ismjth should U> applied only to patelliform -h>lls in which it refers 

 to the di-tanc.> from the anterior to the posterior margin. A whorl represents 

 a sin K rl,> .-oinplete 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 

 numlier 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 

 apei gives the total number. The apical angle, which i> t lie angle formed by the 

 di\ fr^iii^r 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 upon 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. Hut 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 Pahi-ontologie," 

 embraces all the Mollusca which are provided with a tongue in one class, the 

 Glossophora. These are divided into four subclasses, the Seapkepoda, I'lucophora, 

 Gastropoda, and Pteropodn. 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 Bnmeluef&tropoda and the I'ulmogastropoda. 

 The ItrnnchtiHjitstropoda again he divides into four orders, the Prosobrnnrhintn, 

 OpistliolmiiH-hiiifii. I'lernfnula and Heteropoda. Try on, in his great work on "Structural 

 and Systematic Conchology." arranges the same organisms as follows: class 1'trro- 

 poda; class Gastropoda, subclasses Prosobrnnchiatn, Opi$titobnuidliata atnl 1'nhnnnifera; 

 class Scaphopoda. Fischer's arrangement again is different: class I'teropoda; clam 

 Gastropoda; subclass Uni ;/rm, orders /'M/< Oputkobnutckiata, NucUobruw' 



and Prosobranchiala ; subclass Miillii '<///</. order /' 



If we should accept any of the foregoing arrangements it would be /ittel's. l>nt 

 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 di\i-ion- Perhaps a more 

 M 



