1890.] NATURAL SCIENCES OF PHILADELPHIA. 177 



pseudo-ambulacral fields and two intermediate areas." The speci- 

 mens from Kentucky are nearly all attached to Pterofocrinus — P. 

 acutus Wetherby, P. bifurcatns Weth., and P. depressns Weth. In 

 the first of these species the vault is very nuich elevated, being nearly 

 three times the height of the dorsal cup. The first radial dome 

 })lates are produced into monstrous, alate processes, leaving only a 

 small summit which is perforated for the anus. The margin of the 

 growing gasteropod shell has followed closely the surface in contact ; 

 and in the majority of examples when the summit of the crinoidal 

 vault Avas not sufficiently extensive for the suppoit of the enlarging 

 shell, the apertural margin has been prolonged into the interradial 

 depressions, forming prominent, rounded, linguiform extensions ; 

 while the pi'otruding, rounded upper edges of the alate dome-plates 

 of the crinoid have given rise to five deep, broadly rounded sinuses 

 in the lip of the molluscan shell. The lines of growth in the shell 

 are therefore extremely sinuous, the undulations in the direction of 

 the aperture being concave on the broad flattened folds, and convex 

 in the furrows. In some si^ecimens the furrows and folds have their 

 origin very near the apex ; a fact which is suggestive that tlie form 

 of the shell and the configuration of the apertural margin may not 

 have been entirely dependent upon the immediate surface of contact ; 

 but from a long continued habit of adhering to a crinoid presenting 

 such remarkable ventral features as Pterotocrinus, the gasteropod 

 gradually acquired after many generations, a decided tendency to- 

 ward the quinquelobate form, which made itself manifest at an early 

 period of the mollusk's existence, and perhaps even in the latter part 

 of the embryonic stage. 



In order to bring the mouth over the ventral aperture of the 

 crinoid and at the same time rest securely on the limited, flattened 

 summit at one side of which the anal opening was situated it was 

 necessary for the gasteropod to have the anterior portion of the shell 

 directed toward the posterior side of the crinoid — one of the few 

 instances of the kind that has been noted, for almost invariably the 

 front of the gasteropod shell is diiected toward the anterior side of 

 the echinoderm. 



Capulus parvus Swallow. [Plate II, figs. 14a, 14b, 14c.] 



Capidus pai~fus Swallow, 1858. Trans. St. Louis Acad. Sci., vol. I, p. 20-5. 



Platyceras nedrascense Meek, 1872. U. S. Geol. Sur. Nebraska, p. 227, pi. iv, 

 %. 15. 



Platyceras nebrascense White, 1875. E.xpl. W. 100 Merid., Vol. IV, p. 159, pi. 

 xii, fig. 5. 



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