APPENDIX-PATELLILLE. 171 



in all groups of Patellidce; in his recognition of the fact that the 

 primary division of the family is into two groups, based upon the 

 number of anterior side teeth a generalization which I had made 

 before seeing his publication, as will be seen by my synopsis on page 

 79 of this volume. 



I am satisfied that no generic distinctions can be based upon the 

 degree of development of the rhachidian tooth. It varies in differ- 

 ent species from a mere rudiment to a tooth as well-developed as the 

 laterals. It is to be expected that the examination of more material 

 will bridge such gaps as still exist in the range of its variation. 



Dr. Thiele's arrangement is as follows. I have quoted in brackets 

 the species investigated by him of each group. 



Family Patellidce. 



Subfamily PATELLIN.E [equals in limits and contents, division A of 



my synopsis on p. 79.] 



Genus Ancistromesus Dall. [P. chitonoides, P. pica.] 

 Genus PateMidea Thiele. [P. granularis.] 

 Genus Patellona Thiele. [P. granatiua, P. plumbea?, P. adan- 



soni.] 



Genus Olann Ads. [P. cochlear.] 

 Genus Cymbula Ads. [P. compressa.] 



Genus Patellastra Monts. [P. lusitanica, P. guttata, P. ferruginea.] 

 Genus Patella L. [P. tarentina, P. crenata, P. coerulea, P. 



scutellaris, P. aspera, P. lugubris, P. moreleti, P. vulgata.] 

 Genus Patellopsis Thiele. [P. ? shell unknown.] 

 Genus Helcion Moutf. [P. pectunculus Gm. P. pectinatus 



Born.] 



Genus Patinastra Thiele. [P. pruinosa.] 

 Genus Patina Leach. [P. pellucida, P. tella.] 

 Subfamily XACELLIXJE [Equals in limits and contents, division B 



of my synopsis on page 79.] 



Genus Xacella Schum. [P. vitrea, P. hyaliua, P. mytilina.] 

 Genus Patinella Dall. [P. deaurata, P. venosa, P. fuegiensis, P. 



atramentosa.] 

 Genus Helcioniscus Dall. [P. toreuma, P. amussitata, P. testu- 



dinaria, P. exarata, P. rota.] 



Different authors entertain such diverse views upon the amount 

 of divergence sufficient to give generic rank to a group, that any 

 discussion of this matter would be futile. Such differences of 



