a*.) 



; /'. 

 VI. Sulu-la I'KOSOBKAN. in M \. M. IMwank 



. nd organs of sense well developed. A uimals breathing by means of gills 

 .11- l.r.in.-lii.i -ituatt'.l in front of the heart; mostly marine, and provided with a 

 -piral >hll. and generally an operculum. Sexes separate. This is the largest and 



' 'h\ i-i"ii of the class. 



Order Xuckobruw r Htteropoda; foot laterally compressed, with fin-like 



-wimming lobes. Shell sometimes wanting; when present it is more or less symmet- 

 rical, involute and very thin. 



Order /V//ni7;r.in<-/nVi/.i.- branchia> pectiniform, better and more constantly 

 developed than in preceding order; shell spiral, not symmetrical. 



VII. Subclass PULMONATA, Cuvier. 



Annual breathing by means of a pulmonary chamber or lung instead of gills. 

 Sexetf united in the same in-livi-lual. 



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. 



Ob* GASTROPODA. 



Subclass DOCOGLOSSA. 



Order PKOTEOBRANCH I A . 



Suborder PATELLACEA. 



Family PATKLLIDJB. 



The Paleozoic shells which are usually placed in this family are an exceedingly 

 iithVult 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 Acmata. These difficulties are of com - 

 largely due to the imperfect condition in which the shells are preserved. But, even 

 when the muscular scars are retained. an<l tin- i- all we can expect to learn of the 

 internal and soft parts of the animal-, it i- not \>y any means easy to decide ju-t 

 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. 



