112 LECTURES ON MOLLUSCA. 



ning from the green sand. They are generally attached on one side; 

 but the Caribbean Arcinella has the valves furrowed like a Cockle, 

 and attached by the right beak. Fossil Chamas are found from the 

 green sand upwards. One very singular group, Diceras, from the 

 oolite, is like an exaggerated Arcinella. Both of the beaks are prom- 

 inent and spiral, and the muscular impressions are hounded by shelly 

 plates, as in Cucullaea. In the cretaceous Monopleura, the attached 

 valve is funnel-shaped, and the other flat. Another cretaceous form, 

 Mequienia, has the left valve so developed spirally that it has the gen- 

 eral appearance of a Paludina, the other valves looking like a spiral 

 operculum. 



Family HIPPURITIDJE. 



The Rudistes, as Lamarck called them, are characteristic of the cre- 

 taceous age, and are far more aberrant even than Requienia. As there 

 are no living shells at all resembling them, and many of the forms are 

 only known by casts, there has been a great difference of opinion as to 

 their true relations. They were however probably related to the 

 Chama group. In Woodward's Manual, pp. 279-289, will be found 

 an elaborate explanation and figures of their chief peculiarities. They 

 have a general resemblance to Monopleura, having one very long valve, 

 with numerous partitions as the creature advanced upwards, Chamoid 

 teeth, a strong internal cartilage, and tubes in the outer layer of the 

 shell. The free valve is limpet-shaped. The Hippurites cornu- 

 vaccinum is twisted like a cow's horn, and sometimes more than a 

 foot in length. In fiadiolites, the cavity for the animal is much larger 

 in proportion, the internal mould having been called from its shape 

 " Birostrites." Biradiolites has a very large ligamental groove. 

 Caprina has a shape presenting an evident analogy to Requienia. 

 One valve is twisted into a flat spiral, like an Ammonite, and is some- 

 what regularly chambered; the other valve being Hipponyx-sh&ped.. 

 Coprinella has the whirls separated, like Crioceras. They sometimes 

 measure a yard across. Caprotina presents a more normally Chamoid 

 appearance. 



Family TRIDACNIM. (True Clams.) 



The American appropriation of the word "Clam" to the very dis- 

 similar J/?/aand Mtrcenaria is somewhat perplexing, the name having 

 been first given to the ponderous bivalves which inhabit the coral 

 lagoons of the Pacific islands. They have a general resemblance to 

 transverse Cockles, but differ from all other bivalves with closed man- 

 tles in having only one stout adductor muscle, like the oysters; the 

 other being obsolete. The compact mantle has three openings; one 

 in front, for the fresh water ; one near the posterior side, armed with 

 a tubular valve, for escape; and a very large one near the beaks, cor- 

 responding with a large gape in the shell for the finger-like foot, which 

 is grooved to spin a stout byssus. A pair of valves of Tridacna gigas, 

 measuring two feet across and weighing five hundred pounds, are used 

 for holy water in the church of St. Sulpice in Paris. Such a mollusk 

 may have been, when captured, more than a hundred years old. The 



