80 Dentition of Puhnonate Mollushs. 



side ones, g, h), and each cusp is continued into a cutting point 

 (i, J, h). This last I have shaded in my figures, for the purpose 

 of distinction, but no shading exists in nature. The lateral 

 teeth differ from the central by the suppression of the inner 

 lower angle of the base of attachment and the inner cusp and 

 cutting point of the tooth. Thus they are asymmetrical. As 

 they pass outward, they become modified into marginals by the 

 comparative enlargement of the reflection and diminution of 

 the base of attachment, and by the splitting of each cutting 

 point, as well as by the much diminished size of the whole tooth. 

 This form of dentition is, as seen in my description, character- 

 ized by quadrate marginal teeth, from which the other large 

 division is distinguished by having marginal teeth of a strictly 

 aculeate form (see pi. II, fig. H, three left-hand teeth). For 

 the purposes of classification, this distinction of quadrate and 

 aculeate marginal teeth is most important. It sometimes occurs 

 that the central (see fig. K, of pi. XVII) is missing, or the laterals 

 are missing (fig. H, pi. XVII). When the dentition does not agree 

 with either of these two forms, I have considered it abnormal, 

 and described its characters, unless the genus is found in the 

 United States, when I simply refer to my descriptions in Terr. 

 Moll. U. S., V. It must be remembered, however, that there 

 is some variation found from the tooth referred to on pi. VII ; 

 the side cusps and cutting points, especially, being in some genera 

 obsolete (see all the figures on plate IX.) 



A complete list of the lingual membranes examined by me is 

 given in Bull. M. C. Z., Vol. V, No. 16, pp. 339—350. With 

 the original description will be found the name of the person 

 furnishing the membrane. Those from the West Indies were 

 identified by Mr. Thomas Bland. The mounted lingual mem- 

 branes will be found in the Museum of Comparative Zoology at 

 Cambridge. 



W. Gr. BlNNEY, 



Burlington, New Jersey. 



