UNIVALVES. 55 



With another group of three, in which the crown is exactly 

 central, we can end the cap-shaped genera, so far as Britain is 

 concerned. The three are Lepeta, which is white inside and out, 

 and has no shelf in its mouth ; Propilidium, which has a shelf and is 

 oval ; and Calyptrcea, which has a spiral diaphragm within it, and is 

 round and low-crowned, like a Chinaman's hat. 



We may as well tabulate this cap-shaped thirteen : 



Oblong 



Pillar fold broad Testacella. 

 Pillar fold indistinct Ancylus. 



Beak twisted 



Mouth with septum Crepidula. 

 Mouth without septum Capulus. 



Shell slit- 

 Slit at crown Fissurella. 

 Slit on margin Emarginula. 

 Slit between crown and margin Puncturella. 



Crown central 



With spiral diaphragm Calyptrcea. 



With shelf Propilidium. , 



Without shelf or diaphragm Lepeta. 



Crown nearly central Patella. 



Crown near anterior margin 



High and streaked with blue Helcion. 

 Low and not streaked Tectura. 



Next we will take the shells that have a toothed margin ; two of 

 these are sinistral, that is, have left-handed spirals, and one of them 

 has a clausilium, whence its name Clausilia, while the other, Balea, 

 has not the clausilium being an internal plate attached to the pillar 

 of the shell, which can be used to close the entrance, and which 

 differs from the operculum in not being attached to the animal's 

 foot and thus protruded beyond the shell. Two others are almost 

 cylindrical in shape, and look like little beads, these being Vertigo, 

 with an angulated mouth, and Pupa, wrth a mouth that may be ovate 

 or lunate, but is never angulated. One of the Helices, Helix obvoluta, 

 may be mentioned next, the only Helix that has a tooth. Then we 

 have three oval shells Cochlicopa, in which the mouth is pear- 

 shaped ; Carychium, in which it is obliquely oval ; and Melampus, in 

 which it is narrow and the pillar has folds. Of these, Carychium is 

 transparent and the others are not, except one greenish variety of 

 Cochlicopa, and Cochlicopa can at once be distinguished from 

 Melampus by the absence of folds on the pillar. 



We will now take the flat shells that are coiled like a rope on a 

 ship's deck. Three of these have a circular mouth. In one, Homa- 

 logvra, the mouth clasps the periphery ; in another, Planorbis, the 

 mouth is thickened with a rib ; and the other, Valvata, has no rib. 

 The rest of the flat shells belong either to Helix or Planorbis ; those 

 with a mouth of four-fifths of a circle without a rib, or quadrangular 



