270 LECTURES ON MOLLUSCA. 



forms, with square pits, has been used by different authors in such 

 various ways that it may be convenient to revive the old name Isogno- 

 mon, (or Melina.) In some of the tertiary fossils, the pearly layer is 

 an inch thick. Crenatula has the pits small and rounded. In the 

 fossil Gervillia and Bakewcttia, which abound in the secondary strata, 

 there are long hinge teeth inside the ligament row. Inoceramus, 

 which is very characteristic of the cretaceous age, has the shell and 

 the hinge rounded. Some species are a yard long. Other fossil forms 

 are Hypotrema, Catillus, Pulvinites, and possibly Pachymya. 



Family Pectenid^e. {Fan-Shells, or Scallops.) 



The Fan-shells are at once recognized by the broad ears on each 

 side of the beaks, with a slit in one valve for the passage of the foot 

 and byssus. The animals have a double edge to the free mantle; the. 

 inner hanging like a fringed curtain, the outer bordered with a row of 

 minute eyelets, each of which is protected by filaments. The gills are 

 extremely delicate, and hang loose. The lips are beautifully cut. The 

 shell consists almost entirely of membranous plates laid over each 

 other. In the young state all the species moor themselves by a lyssus, 

 which some do permanently. Others live freely, either few together, 

 or in great scallop banks. They can swim by flapping their valves, 

 often jerking themselves some yards at once. They do not abound on 

 the west coast of the Atlantic; but in most seas they are numerous, 

 and generally very highly sculptured and painted ; the lower valve 

 often having a very different hue from the other. Mollusk-eaters con- 

 sider them great delicacies. The cartilage is in an internal pit. The 

 typical Pectens have the valves nearly equal. In Amusium one is gen- 

 erally larger than the other; the shell gapes at the sides; and the 

 valves are either smooth or irregularly waved. In Janira, which in- 

 cludes some of the finest species of the tribe, one valve is flat or even 

 concave, while the other bulges. The J. jacobaia of the Mediterra- 

 nean was formerly worn by pilgrims who had been to the Holy Land. 

 Pallium differs from the ordinary Scallops in having teeth on each 

 side of the hinge-plate. Neithea differs from Janira in the same way. 

 Hemipecten has only one ear ; the other being incorporated into the 

 shell. Fossil species are plentiful in all ages from the carboniferous. 

 Those of Aucella and Aviculopecten form the transition to the Avi- 

 culids. 



Family Limine. 



The Lima group differ from the true Pectens in having no eyelets 

 on the outer mantle-margin, and in having the inner fringed with very 

 long and numerous tentacles. The shells are always white ; and the 

 inner layer is pierced with a network of minute tubes. The ligament 

 is in an external pit, like Vulsella, and the ears are very small. The 

 creatures can swim by jerking their valves, like the Pectens. They 

 either live free, or moor themselves by a byssus; or make a nest of 

 stones and broken shells, spun together by byssal threads, in which 

 they completely hide themselves. Fossil species are extremely nume- 

 rous, from the carboniferous age ; and abound in the Lias and oolites, 



