GASTROPODA. 819 
Patellide. ] 
Division b. 
VI. Subclass Prosoprancurata, M. Edwards. 
Head and organs of sense well developed. Animals breathing by means of gills 
or branchiz situated in front of the heart; mostly marine, and provided with a 
spiral shell, and generally an operculum. Sexes separate. This is the largest and 
most typical division of the class. 
Order Nucleobranchiata or Heteropoda; foot laterally compressed, with fin-like 
swimming lobes. Shell sometimes wanting; when present it is more or less symmet- 
rical, involute and very thin. 
Order Pectinibranchiata; branchie pectiniform, better and more constantly 
developed than in preceding order; shell spiral, not symmetrical. 
VII. Subclass Punmonata, Cuvier. 
Animal breathing by means of a pulmonary chamber or lung instead of gills. 
Sexes united in the same individual. 
A classification of the Paleozoic genera of the Gastropoda and remarks on their 
geological distribution, especially of the Lower Silurian types, followed by a summary 
of the principal results of our work, will be found at the close of the chapter. 
Clas GASTROPODA. 
Subclass DOCOGLOSSA. 
Order PROTEOBRANCHLA. 
Suborder PATELLACEA. 
Family PATELLIDA. 
The Paleozoic shells which are usually placed in this family are an exceedingly 
difficult group. While we may be reasonably confident that the relations of some 
of them are not far from the recent genus Patella, there are many others that 
remind one quite as much of Lepeta and Acmea. These difficulties are of course 
largely due to the imperfect condition in which the shells are preserved. But, even 
when the muscular scars are retained, and this is all we can expect to learn of the 
internal and soft parts of the animals, it is not by any means easy to decide just 
what affinities they indicate most, because these scars, like the whole form of the 
shells, are in a general way very similar among the twenty or more recent patelloid 
genera and subgenera. 
