262 LECTURES ON MOLLUSCA. 



Mbntacuta is destitute of the anterior tube, and the shell is slanting; 

 the cartilage occupying a pit between two strong teeth. Pyihina has 

 the shell narrowed in the middle, generally with slanting sculpture. 



Family Leptonhle. 



This group differ from the Kelliads in having the mantle produced 

 beyond the edge of the valves, and adorned with filaments. The foot 

 is spread out, for crawling like a Gasteropod. Lepton has a shell 

 somewhat resembling Kellia, often minutely punctured, with diverging 

 teeth. Tellimya resembles Montacuta in shape, but has an ossicle in 

 the cartilage-pit, like the Anatinids. Galeomma resembles an arc, 

 with a wide gape in front. It has a small cartilage-pit, without teeth, 

 and opens its valves wide, like Solemya. Scintilla has small hinge- 

 teeth, and gapes at the sides. Cydadella perhaps belongs to the 

 same group ; but has la'teral teeth, a hinge-tooth parallel to the margin, 

 and an external ligament. 



Family Solemyadje. 



The little group called Solemya appears more related to Galeomma 

 than to either Solen or My a. The shell is extremely thin, enclosed in 

 a wide horny skin. The hinge resembles Leptom, with a very long 

 cartilage-pit. The creature opens its valves very wide, and swims 

 by dilating the end of its wide foot, which it works as we open and 

 shut an umbrella to shake off the wet. The mantle is closed in 

 front ; and there is a tail on each side of the excurrent opening. There 

 is only one gill on each side. 



We now proceed to the freshwater families; the first of which has 

 relations both to the Kelliads and the Venus tribe. 



Family Cyrenid^:. (Fresh-water Cockles.) 



These creatures hatch their eggs within the mantle, but are not very 

 prolific. Their habits may easily be observed by placing the little 

 creatures, which may be found in any pond or ditch, in a little fresh 

 spring water. They then drag themselves along by extending their 

 transparent tongue-shaped feet, and protrude their short pipes. The 

 young shells are sufficiently transparent to allow of the gills and heart 

 being seen within. Cyrena has two short, separate pipes, and a strong 

 shell, with 3-3 hinge-teeth, and smooth laterals. It is found in the 

 English tertiaries, but is now confined to tropical regions. Corbicula 

 has furrowed valves, with grooved side teeth. The shells of Batissa 

 have strong hinge-teeth, with very short laterals. They are from the 

 Pacific islands. Velorita has a very stout hinge, somewhat resembling 

 Cyprina, with a slight siphonal fold. 



The temperate regions abound in the thin shells of Cyclas, which 

 has two rather long pipes, partly united ; and of Pisidium, in which 

 the shell is slanting, and there is only one excurrent pipe. Both Cyclas 

 and Cyrena are found fossil as far back as the Wealden' rocks. 



Family Unionhle. (Fresh-water Mussels.) 

 As far as shells are concerned, this family forms the special glory of 



