GASTEROPODA. 85 



the aperture of the shell is entire ; in others it is interrupted, 

 the left side being formed only by the "body-whorl :" or the 

 " peristome" (as the margin is called) may be broken by a 

 notch, like that which separates the outer lip from the 

 umbilicus ; or it may be perforated by one or more holes ; or 

 a portion of it may be produced into a canal or siphon; 

 this (fig. 25, ac,) is sometimes termed the "anterior canal," and 

 the notch or hole at the opposite end of the peristome is called 

 the "posterior canal" pc. These modifications are important, 

 on account of the constancy of their relations to certain con- 

 ditions of the respiratory organs. Thus all the pectinibranchi- 

 ate Gastropods, in which the water is conducted to the shell 

 by a muscular tube or siphon, have the margin of the aperture 

 either notched or produced into a canal, ac ; the posterior 

 one, pc, is anal in its function (Triton, Strombidce) : sometimes 

 it is represented by a slit (Scissurella), or it is a tube {Typhis), 

 or a perforation (Fissurella), or a series of holes, as in Haliotis. 



The relations of these modifications of the univalve shell, 

 which anatomy has made known, enable us to judge in a 

 general way, from a fossil shell, of the sphere of existence, of 

 the respiratory medium, and to a certain degree of the food 

 and habits, of its extinct constructor. The Gastropods, which 

 first appear in the Palaeozoic strata have entire mouths ; the 

 siphonated species are not found lower than the Lias, and 

 they go on increasing in numbers in and from the Tertiary 

 series to the actual sea-shore. 



Fossil univalves — the remains of spiral and limpet-like 

 shells — are not wanting in any but the very oldest fossili- 

 ferous rocks ("lingula flags"). From the lower Silurian, 

 where less than 100 species referable to scarcely more than 

 ten genera, are found, they increase in number and variety 

 slowly and regularly up to the newer tertiaries, which have 

 afforded ten times as many genera and twenty times as many 

 species. The total number of fossil marine univalves is less 



