LECTURES ON MOLLUSCA. 271 



where they are often of large size, and are called Plagiostoma. Idmcea 

 begins with the Lias, and has one recent representative. It is a Lima 

 with a row of Area-like hinge teeth. Limatula, a northern group 

 which begins in the English Crag, has the valves equilateral. 



Family Spondylid^e. (Thorn-Oysters.) 



These creatures may be regarded as attached Fan-shells ; and form 

 a natural transition from them to the true Oysters. The animal of 

 Spondylus closely resembles that of Pecten, but the foot is rather more 

 rudimentary, and there are no eyelets. The shell has strong inter- 

 locking teeth, and the attached valve has a very long beak, with a flat 

 area, which is wanting in Plicatula. In one specimen of the " Water- 

 clam" (so called from the layers of shell having spaces between them) 

 in the Smithsonian Museum, there is an area in both valves. Fossil 

 species are found from the lower oolites. The Spondylus spinosus, a 

 very characteristic species of the chalk, lived nearly free ; like the 

 recent S. imperialis. Hinnites begins free like a Pecten, and after- 

 wards becomes fixed. Pedum has a thin, flat shell ; living imbedded 

 in madrepores. It has a deep notch for a byssus in the lower valve. 



Family Ostreid^e. (Oysters.) 



As all readers of this report have access to Oysters, which, instead 

 of eating, they can dissect and examine at pleasure, it is needless to 

 describe either the shell or the animal. The chief peculiarity is the 

 entire absence of foot. They are found in all seas, and in every age 

 from the carboniferous ; varying greatly in form, according to the sur- 

 face to which they have been attached. The mangrove-oysters (Den- 

 drostrea) are thin and but slightly attached. The cock's-comb spe- 

 cies are deeply plicated. In the fossil genus Gryplma one valve is 

 spirally twisted, and the other nearly flat. The animal was probably 

 not attached. The shell, of Exogyra, characteristic of the oolitic and 

 cretaceous ages, is CAama-shaped. The fossil Ostrea longirostris of 

 the Tagus is sometimes two feet long. 



Family Placunidje. (Window-Shells.) 



The Placunids are extremely flat, thin creatures, with a very unu- 

 sual hinge. There are two long divergent teeth, like a V, to the sides 

 of which the ligament is attached, as in Pandora, to which the shell 

 offers some resemblances. It consists of very thin, somewhat nacre- 

 ous plates. The shells of Placuna, often called Saddle-oysters from 

 their shape, have the hinge-ridges equal, and rapidly diverging. 

 Those of Placenta are nearly transparent, being used lor window glass 

 by the Chinese ; and have the hinge ridges nearer, and one shorter 

 than the other. Placunopsis is an oolitic fossil, with a transverse liga- 

 ment groove. There is only one principal muscular impression in the 

 Placunids. 



Family Anomiad^:. 



The shells of this family are remarkable for the large number of 



