No. 2.] 



SNAILS OF THE GENUS 10— ADAMS. 65 



Turrita represents the extreme degree of specialization, or departure from the ancestral 

 form, in its elongate shell, small aperture, degree of spinosity, and early age at which spines are 

 developed. It has, as it were, abbreviated its ontogenetic development of spines so much that 

 spines develop very early in life, and not relatively late, as in its associated form loudonensis. 

 It is remarkable that we can find living to-day what appear to be several stages or kinds of 

 development from the smooth to the sculptured shells, not only in a single stream but also 

 duphcated in other streams. This naturally raises the question as to which transition most 

 nearly approaches the ancestral one: Were the ancestral ones duplicated also or have all of them 

 been formed by combinations of relatively independent characters and are lacking of much 

 phylogenetic significance? If ontogenies are modified by combinations, a decision must be 

 made as to the relative values of each one. 



The perfection of the transition seen in verrucosa, and the attenuation of undulations with 

 departure from the Upper Holston, and its inverse development suggests its independent charac- 

 ter. A similar independence of development is seen in loudonensis, but each seems lacking in 

 both lyttonensis and paulensis, and these may represent new or later combinations. In view 

 of the probabihty that spinosity developed farther south, loudonensis may give us the best idea 

 of the transition from smooth to spinose shells, and verrucosa the best idea of transition from 

 smooth to undulate, on the one hand, and from undulate to spinose. 



GENERAL DISCUSSION AND SUMMARY. 



1. INHERITANCE IN MOLLUSKS. 



Early in my studies of these shells I had planned for hybridization experiments which were 

 to be conducted after some perspective had been acquired by a preliminary study of the shells 

 from many localities. Live specimens were secured in 1900 and kept alive for some months, 

 but proper aquaria were not available, so that this important phase had to be abandoned. 

 Some one with the faciUties of a trout hatchery in southwestern Virginia or eastern Tennessee 

 ought to be able to secure valuable results from this kind of experimentation, as this family 

 of shells is one of the most plastic and characteristic groups of North American animals. Bartsch 

 ('06, p. 465) has proposed a series of transplantation breeding experiments upon a member of 

 this family belonging to the genus Goniohasis. It should be added that pedigreed material, 

 or pure strains from nature, should if possible be used in such an experiment. 



Very little is known in general of inheritance in gasteropods. A few references have been 

 found in the literature, in which the parent and young of viviparous species have been compared, 

 although it is probable that many of such cases are scattered about in conchological literature. 

 Mayer ('02, p. 121) observed in the hermaphroditic and viviparous land snail Partula, from 

 Tahiti, that: 



The young of dextral or sinistral snails are usually dextral or sinistral respectively, but this is not invariably the 

 case. * * * All of the young developed within any given adult are either dextral or sinistral, never some of them 

 dextral and others sinistral. 



The significance of this form of inheritance is shown in a further statement that — 



All the snails of Tipserui Valley are dextral, while all of the same species in Pirae Valley are sinistral. In the 

 two intermediate valleys of Hamuta and Fautaua some individuals are dextral and some sinistral. 



Bartsch ('07, p. 146) has observed the inheritance of shell sculpture in the variable fresh 

 water snail Vivipara lanaonis Bartsch. He says : 



It is an interesting fact that in all the gravid specimens examined, the nepionic shells taken from the parent always 

 had the sculpture of the parent. 



Coutagne (1894, 1896) has studied inheritance in Helix, but his work is not accessible to me. 



The most important and prolonged studies in inheritance in molluscs have been made by 

 Lang (1904, 1906, 1906a, 1908). I have only had access to a part of Lang's papers, and abstracts 

 of the others (Darbishire, 1905). The following observations pertain to the present problem; 



