238 SYNOPTIC COLLECTION. 



lamarcki P. & L., in which the early tendency to coiling 

 is seen at the apex, while No. 540, C. cithara Bens., is 

 suggestive only of the cap-like form. No. 541 is the 

 fleshy animal (C. mediterraned), and PI. 542, a drawing of 

 C.fragilis Bory. It is drawn with the ventral side upper- 

 most, the position it often takes in swimming. The body 

 is long and the leaf-l:ke fin (see No. 541) which is a modi- 

 fied foot is seen above, while below, the visceral mass con- 

 sisting of liver, kidney, heart, reproductive organs, etc., 

 is reduced in size and covered by the small shell (No. 



540- 



The more specialized Heteropods are represented by 

 Pterotrachea (No. 543, P. mutica\ No. 544, P. coronata-, 

 see lowest shelf) in which the body has lost its spiral 

 form and become cylindrical, while the shell has wholly 

 disappeared. The tentacles are also lost and the gills 

 exist only as vestiges. 



PTEROPODA. 



Recent researches point to the conclusion that the 

 ancestors of our present Pteropods existed in the early 

 Palaeozoic times. 



A near ally of Tentaculites, Orygoceras (PI. 545, fig. i, 

 O. dentaliforme ?), had a plain unornamented shell. The 

 protoconch is not represented in the figures of this spe- 

 cies, but is finely shown in O. cornucopias (PI. 545, fig. 2). 

 Succeeding the protoconch the young shell of this species 

 is seen to be unornamented like the adult (fig. i). As 

 the animal grows older the shell becomes ridged by many 

 circular rings (fig. 2). 



Tentaculites (No. 546, T. gyracanthus Eaton; PI. 547, 

 T. acuarius Richt.), has a straight, cone-shaped shell like 

 that of Orygoceras, with a simple circular aperture, and 

 with concentric ridges of rings extending around the out- 

 side. These ridges are carried back nearly to the proto- 



