VIII SUMMARY. 



MOLLUSCA. Mauy of the families of this order are destitute of a 

 shell : the following characters will direct to the Families and 

 Genera in which the native species are contained : 

 The shell formed in separate pieces, placed transversely across the 

 dorsal surface : Chiton. 



The shell of one depressed or conical piece, destitute of convolution. 

 In one or two of the genera, the summit is a little bent, and in another, 

 the internal cavity has a spiral plate: comprizing the Genera Patella, 

 Emarginula, Fissurella, Pileopsis, Calyptraea, Ancylus. 



The shell inflated, aperture wide, the length of the shell, no Columella 

 or projecting spire : Bullaea, Bulla. 



The shell thin, depressed, the spire short, last whorl large, aperture 

 large, oval : Vitrina. 



The shell depressed and earshaped, the spire small, aperture wide 

 oblong, margins separated: Sigaretus, which is distinguished from 

 Natica and Neritina, by its more depressed form, by the width of the 

 aperture, and the want of the umbilical callosity. 



The shell spiral somewhat depressed, aperture rounded, the margin 

 disunited, by the projection of the largest whorl without a canal : Helix, 

 Zonites, Succinea. 



The shell spiral, elevated, aperture entire, the margins disunited by 

 the projection of the largest whorl, no canal : Bulimus, Zua, Pupa, 

 Vertigo, Balea, Carychium. 



The shell spiral, elevated, slender, aperture irregular, with the borders 

 united, the lowest whorl not the largest : Clausilia. The other forms 

 of spiral shells, without a canal, will be found in the Genera Cyclostoma, 

 Limneus, Nerita, Natica, Ianthina, Tornatella, Marginella, Voluta, 

 Terebellum, Turbo, Scalaria, Trochns, Cingnla, Phasianella, Turritella. 

 With the spire very much depressed, Planorbis. Skenea. 



Spiral shells, with a canal leading from the aperture : Cerithium, Pleu- 

 rotoma, Fnsus, Triton, Purpura, Buccinum, Terebra. 



Spiral shells, with a canal and an expansion into a wing of the 

 outer border of the aperture : Rostellai ia. 



The shell oval, spire scarcely visible, aperture longitudinal, narrow, 

 toothed on both sides, the extremities effuse : Cypraea. 

 CEPHALOPODA, or CUTTLEFISH TRIBE. Some of the families 

 of this order have no shell ; others have it only internal. In the 

 family of Orthocerata, the shell is straight, or nearly so, with in- 

 ternal chambers, perforated by a tube: Orthocera; easily to be 

 distinguished from Vermetus, and Serpulaceas, by the internal 

 divisions. As is also the genus Spirula which in outward form 

 approaches to the shape of Spirorbis, but in which the volutions do 

 not touch each other. 

 The characters of the separate Genera will be. found in the body of 

 the work. 



