252 



LO WEU INVERTEBRA TES. 



G. 25S. — TroeliosplKTc 

 of oyster; f. fool; m, 

 niou'tli; a, sliell gland; 



<:. 259. — Veligei- of Oiiistli 

 braiicli; ((, anus; /, foot; 

 intestine; o, operculum; 

 velum. 



of many worms, and t'Sjiecially tlie rotifers. At this stage, or even earlier, another 

 iiiiportant feature aiipears, tlie shell gland. Tins is at first an invagination on the side 

 of the body opjiosite the mouth, but 

 still outside the velar area. The gland 

 soon flattens and begins to secrete the 

 shell, which at first appears as a single 

 delicate plate. Following tlie trocho- 

 sjjhere conies the stage known as the 

 veliger. The velar area is now a flat- 

 tened plate, fringed with cilia, and fre- 

 quently expanded into lobes, while the 

 rest of the bodj' is greatly enlarged 

 in proportion. The foot is also more 

 prominent. With subsequent development the disproportion 

 between the velar area and the rest of the body increases 

 in all except a very few cases, as the pond-snail, Limncea. 



?>liells, more than any other objects of natural history, have played a part as objects 

 of merchandise, and for the rarities, conchologists have in times past paid the most 

 fabulous prices. The following, copied from Tryon, may prove of interest to those 

 who liuve not yet caught the fever of shell-collecting : — 



'" Scularia pretiosu, which can now be had for one or Xwo dollars, was worth 8100 

 in 1735, and $200 in 1701. Phasianella buliinoides, which also brought $100, can 

 now be purchased at from one to two dollars, or even less. In 1865 a great English 

 collection, that of Dennison, was sold by auction in London, and some extravagant 

 prices realized. Ci/pi-cea ffuUata hvought, $200; Ci/prcea pri/icejjs, the same; Vonus 

 fyloria-maris, also $200 ; Conus cervits nearly $90 ; Comes cedoimUi (not a veiy rare 

 shell), $90 and $110; Conus omaicus, (also not rare), $60; Valuta festively $80; 

 Oniscia dennisonii, $90; Pholadomyiu cundida, $65; Carinaria vitrea, (wliich Mont- 

 fort stated to be worth $600), brought $50. The very rare Pleurotonia quoyana 

 brought in London, in 1872, $125. In 1876 the Roters van Leimep collection was 

 sold, including: Voluta jimonia, $50; Mitra belcheri, $40; Spotidi/lus regius, $36, 

 etc. For this same Spondijlus regius Professor Richard had previously paid several 

 thousand francs. Yoluta jimonia has always been considered a rare species, and 

 dealers have obtained as much as forty pounds sterling for it. . . . Cypircpa 

 %imbilicata has been sold for thirty pounds, and may now be had for one pound. The 

 Boston Society of Natui'al History possesses an Argonauta wgo^ or paper nautilus 

 shell, which is said to have been purchased by the gentleman who jiresented it to that 

 society, for $500. It is a common species, and the only reason of the greater valua- 

 tion of this specimen is that its diameter is about two or three inches greater than any 

 other individual known to naturalists." 



Class I. — ACEPIIALA. 



This group of the molluscs has been burdened with a large number of names. 

 Among them we find Conchifera, Endocephala, Lipocephala, Lamellibranchiata, and 

 Pelecypoda, as well as the older, and consequently ]ireferable, designation adojited 

 here. The group will readily be recognized by all under the popular designation of 

 bivalve molluscs. In this more familiar name is embodied one of the most character- 



