INSTITUTE OF SCIENCE, PHILADELPHIA. 65 



In the large and thin species such as are usually associated under the 

 form of Cymba and Melo, the axis is almost or quite pervious, and, instead of 

 a central solid axis or pillar, wehave a thickened edge corresponding to a 

 very slender and twisted columella. In these species, according to the rule 

 above cited, the plaits are very oblique. In a few fossil forms, in which, with 

 the general characters of Cyinba, is associated a thick and solid pillar, the 

 plaits are more horizontal, thus according with the theory. 



The existence of a long canal giving a slender shape to the shell does not 

 necessarily imply that oblique in-and-out motion of the animal which would 

 result in oblique plication. If the whorls, apart from the canal, are closely 

 coiled and the canal is somewhat constricted off from the whorl behind it, the 

 in-and-out motion of the animal will be transverse to the axis rather than ob- 

 liquely along it, and horizontal plaits will result, as in Turbinella pyrum, not- 

 withstanding the elongated form which the narrow canal gives to the shell. 



The factors leading to the development and especially those influencing 

 the form of the plaits are complex, and it may be difficult, some may think it 

 impossible, to assign to each its proper weight in producing a given result. 

 But this complexity is a part of the much greater complexity of Nature, and 

 the attempt to interpret it must reckon with the inherent difficulties in any event. 

 There is no short and easy byway to a genuine comprehension of Nature. Yet I 

 think it has been shown with sufficient clearness that dynamic and not selective 

 influences are the chief factors in the minor problem now under consideration. 

 Characters in the Nucleus of Volutidce. — In the group under consideration 

 we are fortunate in having one feature which enables us to understand some- 

 thing of the embryonic or larval history and thereby to partially substantiate 

 the hypothesis as to descent drawn from other characters. I refer to the 

 characteristics of the larval shells. 



In the Mesozoic transition-forms the larval covering was shelly and minute, 

 its upper surface like a minute Comihis or Hyalinia. This indicates that those 

 forms found their benefit in the production of a large number of small shell- 

 bearing larvae rather than in a small number of large ones. This holds good 

 for all the Mesozoic forms, at least in America, and for all the exotic Mesozoic 

 forms I have been able to examine. These small larval shells are persistent 

 and often of a different texture from the succeeding whorls of the adult. 



This, being observed, has been described as a " papillose " apex, a grossly 

 inaccurate use of a term which was originally intended and is still chiefly used 

 to denominate a subglobular, few-whorled, inflated nucleus like that of Tudicla. 

 Indeed, Deshayes and other paleontologists have used a single term such as 

 " mamelon " to denominate the larval shell of all Volutes, regarding the differ- 

 ences as merely of size. He says (An. sans Vert, bassin de Paris, III. p. 582), " La 

 mamelon existe sans exception dans toutes les Volutes, aussi bien dans les 

 fossiles que dans les vivantes, " while allowing them " une coquille embryon- 

 naire quelquefois enorme et d'une forme toute particuliere. " 



