30 GASTROCOPTA, NORTH AMERICA, WEST INDIES. 



stome, more deeply seated in the throat, and occasionally one 

 or two very minute rudiments on the peristome. Length 1.56, 

 diam. .84 mm. This shell is found on both sides of the Ohio 

 Eiver, near Cincinnati." (Judge, The Quarterly Journ. of 

 Conch., p. 343, 1878.) 



The increase in number of accessory denticles or teeth cul- 

 minates in the form named curvidens, represented in pi. 4, 

 figs. 9 to 15, also pi. 5, figs. 3, 6, 32, and perhaps some others, 

 as there is absolutely no line to be drawn between pentodon 

 and curvidens. The number of teeth varies from 5 to 9 in 

 perfectly adult shells of the same gathering from one spot. 

 Gould's type had 9 teeth. It was from Phillips Point, Lynn, 

 Massachusetts. PI. 3, figs. 6 (Ocean City, N. J.), and 7, 8 

 (Troy, N. Y.), are good examples of the form curvidens. 



It may be stated as proven that some colonies consist of 

 "pentodon" and intermediate forms; some of "pentodon," 

 intermediate and "curvidens" forms; and some of the inter- 

 mediate and "curvidens" forms. We have found no large 

 gathering of wholly typical pentodon or entirely curvidens. 

 Either form may be found with a low or high crest this 

 being usually more constant in any one colony than the num- 

 ber of teeth or the size and shape of the shell. The only 

 theory upon which curvidens could be retained as a species 

 or subspecies would be to assume that two species are living 

 in a state of hybridism. 



The development of an infraparietal denticle is extremely 

 variable in many lots, as in that from Buckfield, Me., pi. 4, 

 figs. 1 to 4, 8. This infraparietal tubercle is exactly compar- 

 able to that of some Eastern species, such as G. armigerella 

 of Japan and China. It is not found in other American Verti- 

 gopsis, though occasionally developed in the typical section of 

 Gastrocopta. 



P. florida/tm (pi. 4, fig. 9, a cotype from the author) is ab- 

 solutely identical with curvidens. It is described as "sub- 

 cylindrical, . . . teeth about 9, of which there are generally 

 3 larger than the rest, their tips nearly meeting and their 

 bases mutually nearly equidistant; one is on the pillar, one 

 on the body- whorl, and one on the anterior margin ; on either 



