GASTROPODA. 



[Streptoneura. 



dL-, 



TERMS EMPLOYED TO DENOTE VARIOUS PARTS OF THE UNIVALVE 



SHELL. 



The univalve shell is composed of one or a number of whorls, the 

 whole series of whorls, except the last or body-whorl (b-c), forming 

 the spire (a-b) . A whorl is a single 

 revolution of the spiral cone round 

 the axis. The apex consists of 

 the embryonic shell or proto- 

 conch (d). The line of junction 

 between two successive whorls 

 is the suture (h). The sculpture 

 may consist of spiral lines or 

 ribs (e), and of axial ribs (/) 

 and varices (g), the latter ^ 

 being marginal ribs of the 

 aperture of an earlier stage of 

 growth. The mouth or aper- 

 ture (i) may have a posterior 

 canal or channel (k) and an 

 anterior canal (I) ; on the right 

 side is the outer lip (m), and 

 on the left the inner lip (n) 

 spread over the pillar, or 

 columeUa (o). At the base a 

 a siphonal fasciole (p) and an 

 umbilicus (q) may be present. The height of the shell is^given by the 

 line a-c. 



--I-- W, 



SUBCLASS I. STREPTONEURA, Spengel. 

 (= Prosobranchia, Milne- Edwards ; CochUdes, von Ihering.) 



These are dioecious (i<t*ti'<>poda. with the exception of a few aber- 

 rant genera, and are characterized by the maxium torsion exhibited 

 by the visceral mass and visceral commissure, the latter being always 

 twisted into a figure of eight. The right moiety of this commissure 

 is situated above the digestive tube, and is known as supra-intestinal ; 

 the left moiety is situated below the digestive tube, and is known 

 as infra-intestinal. 



The head of Streptoneura bears only a single pair of tentacles.^jThe 

 radular teeth, when there is more than one on either side of the median 

 tooth, are of several different kinds in each transverse row. The 

 heart is almost always posterior to the branchia. 



The subclass includes two orders, Aspidobranchia and Pectini- 

 branchia. 



