254 LECTURES ON MOLLUSCA. 



"what wedge-shaped, with a bipid tooth in one valve. The animal is 

 of sedentary habits, boring in mud or clay. The shell of Elizia is 

 very like a flat Diplodonta, but there is a wide mantle-bend. Lucin- 

 opsis has a swollen thin shell, with a binge like a Venus; but the 

 animal is of the Tellen type. 



The next group have the cartilage internal, like the Madras; which 

 appears at first sight a very great distinction, but there are some species 

 that might be ranged with equal propriety in either section, the car- 

 tilage-pit being at the margin, close to the ligament, which is always 

 external and generally slender. Scrobicularia lives buried in estuary 

 mud, extending its pipes five or six times the length of the shell. 

 The hinge-teeth are very small. Semele has a stronger shell, with a 

 tooth on each side of the cartilage-pit. Syndosm.ya has a very thin, 

 white, Tellinoid shell ; with a hinge like Scrobicularia, but with lateral 

 teeth. The animal of Gumingia is irregular, the shell being found 

 nestling in crypts like Saxicava. One valve has very strong lateral 

 teeth; the other none. 



Family D on acid^e . ( Wedge-Shells . ) 



The Donax family differ from the Tellens in having shorter breathing- 

 pipes, and stout, triangular shells. In the typical species of Donax, 

 the breathing-end is very short, the foot-end long and pointed. The 

 valves are stout, with crenulated margins and short ligament. There 

 are strong lateral teeth. Heterodonax wants the crenulations, and has 

 a rounded form. Iphigenia has a somewhat swollen shell, without 

 lateral teeth. It lives in estuaries, and the species greatly resemble 

 each other. The curious genus Galatea is peculiar to the African 

 rivers. It has a very thick, triangular shell, with stout hinge-teeth 

 like the Venus tribe. 



Almost every sandy shore in the warmer regions has its species of 

 Donax, which lives in myriads at a certain depth below the surface. 

 At Panama, the natives clear off the sand just below this depth, and 

 thus quickly collect bushels of the mollusks, which are considered 

 dainty food. Yet the species, though more abundant than any other 

 bivalves, are less widely distributed than most, each district having 

 its peculiar form. They have not been found fossil previously to the 

 tertiary ages. As among the Tellens, so here, a group is found with 

 an internal cartilage. The marine Erycina* has no little external 

 resemblance to Galatea, being triangular and. solid ; but the cartilage 

 is in a narrow pit between stout teeth. Mesodesma, which abounds in 

 the Australian region, is shaped like Psammobia, but solid ; with two 

 short, stout lateral teeth. Donacilla has a wider distribution, and is 

 wedge-shaped, with one of the lateral teeth long. Geronia, one spe- 

 cies of which inhabits the New England seas, has the side teeth 

 strongly grooved. The Messrs. Adams unfortunately assign all the 



*The genus Erycina is here restricted to the triangular shells of the Mesodesma type, 

 called P aphid by modern authors. This latter name has a very obscure and intricate gene- 

 alogy, and had better be dropped, as it 'is in use for butterflies. The heterogeneous genus 

 Erycina of Lamarck has very properly been dismembered ; but the name should be kept for 

 the principal species. 



