Hogg, on the Lingual Membrane of Mollusca. 99 



disposed in longitudinal series. In the Pulmonata there is a 

 single tooth in each median row, with a number of broad and 

 similar laterals disposed in rows on each side, Avhile in other 

 groups the teeth are arranged in three, five, or seven dis- 

 similar rows. Since each row is exactly similar to every 

 other, the system of teeth admits of an easy representation 

 by a numerical formula, in which, when the uncini are nu- 

 merous, they are indicated by the sign oc , infinity, and the 

 others by the proper figures. Taking Nerita or Hclicitia as 

 our type, we designate as laterals the broad teeth on each 

 side of the median row, the numerous small teeth on the out- 

 side of the band being termed pleura, and those, still smaller, 

 on this, uncini ; the latter, found only in certain groups, are 

 usually of extreme tenuity, often beautifully outlined, and 

 frequently serrated. 



Dr. Gray's scheme for a classication of mollusca is cer- 

 tainly open to criticism ; and it may fairly be asked if any 

 reliable classification can be got out of a union under one 

 formula of so many families as we find grouped in Tsenio- 

 glossa. Mr. Gwyn Jeffreys, while he expresses a doubt of 

 the value of such an arrangement, admits that the tongues 

 of mollusca " may furnish important characters of such 

 genera as Crepidula, Calyptrsea, Patella, &c., which, from 

 their having been long attached to particular places, change 

 the external character of their shells, and thence assume par- 

 ticular forms, which have been regarded as distinct species." 

 Mr. Wilton satisfied himself that Patella athletica could be 

 distinguished from the common limpet of our coasts by its 

 teeth, and also that a similar difference is seen between the 

 two Cape species, P. apicina and P. longicostata. It will 

 not be said that the incongruous group enumerated under 

 Tsenioglossa, in which the cuttlefish and river-snail are linked 

 together, at all approaches perfection. Undoubtedly it is a 

 strong point against this, or any other mode of classification, 

 that it places together, in an unusual and embarrassing man- 

 ner, carnivorous and phytivorous mollusca, " widely differ- 

 ing in habits and anatomical characters." But it may be 

 replied, that in some classes the general characteristics are 

 equally liable to mislead. Take, for example, the slug 

 family, which is made to include Testacella ; the slug being 

 almost exclusively a vegetable feeder, while the Testacella 

 is one of the most savage of flesh-eaters well known to 

 pursue its prey, the earthworm, in its haunts with intense 

 voracity and cunning. Even the shell affords little or no 

 protection, being in both alike the merest rudimentary struc- 

 ture, serving only the purpose of a shield when the long, 



