322 



ROBERT TRACY JACKSON ON THE 



and PectinidiT?, as proposed by the authors of the 'History of British Molliisca,' has not 

 proved satisfactory. The genus (3strea stands quite ahjne, and distinct from all the Pec- 

 tinidae in the structure of its gills, which are like those of Avicula, and by resting on 

 its left valve. Tiie shell also is more nacreous than that of the scallops." Another 

 leading naturalist, 1 )i'. C. A. White, in a recently published monograph of the Ostreadse 

 of North America, says: " To Avhat primary cause this asymmetry among the Ostreidae 

 is due, it is, with the present limitation of our knowledge, impossible to say; but it is 

 certainly a characteristic of the whole family, including all its genera and its fossil as 

 well as living forms." 



My studies of the Ostreadae, and other Pelecypoda as well, have brought me to a con- 

 clusion in regard to the origin of the peculiar forms we have here to deal with, which 

 is that the ostrean form is due to the conditions of a direct cemvited fiiorition, acting 

 wpon a Pelecypod shell. Given such conditions, and a closely similar form is the result 

 in widely separated genera of this class, as I shall attempt to show. The form which is 

 claimed as the result of the conditions of direct, cemented fixation is a concave attached 

 valve; a flatter free valve, commonly much thinner than the attached valve; an irregu- 

 larity and asymmetry of growth tending to the displacement of characters noi-mally 

 found in near allies of the subject under consideration, and as a general thing a camer- 

 ated structure of the shell. The fullest modification in this line of variation is the 

 production of a shell in which the attached valve is cup-shaped, conical, or sub-cylin- 

 drical as seen markedly in the Chamidfe and Rudista?. In this group all the transitions 

 may be traced from a simple ostreaform or exogyi'iform shell to the most highly inaditied 

 conical type. A further result of the modified condition induced by fixation and which 

 I consider perhaps the most interesting of all, is the fact that the attached valve is the 

 most highly modified; the free valve being the least modified and retaining more of the 

 ancestral characteristics, encMing one through it to trace the genetic connections of the 

 group, evidences of ivMch may he completely eradicated from the attached valve. If the 

 ostrean form is due to the conditions of dii'ect fixation, then all or nearl}^ all Pelecypoda 

 which have one valve cemented to a foreign body for a longer or shorter time should be 

 ostreaform or modified in this line of variation.^ First, fixed forms will be considered as 



'The Rudista' are conical or cup-slaaped Pelecypods witli 

 a superticially marlicd radial syiniiietry. So striking is 

 the radial feature that they have been classed with the 

 corals or Cirrhipeds, and the term radial is combined fre- 

 quently in generic and speciHc names of the group. 

 Barrelia monnlifcra, described by Woodward (78), is 

 highly radial, and infoldings of its thin walls closely 

 resemble the radial septa of corals. In other animals 

 which are permanently attached by calcareous fixation as 

 corals, some worms, and Brachiopods, Cirrhipeds and 

 others we find closely comparable forms in which radial 

 symmetry is a marked feature. When similar forms are 

 built up on different lines of descent, thei'e is strong evi- 

 dence that common forces acting on all alike have in- 

 duced the resulting form. 



I suggest the following as a possible explanation of 

 the cause of these markedly similar forms, and would 

 say that I have a great deal of evidence on the question 

 but it must be reserved for a future iiublication. The 



e(inal impact of moving water on all sides of an attached 

 growing organism it seems would cause an equal effort 

 of resistance on all sides, and therefore induce an equal 

 growth on all sides, thus producing a form circular in sec- 

 tion at any one horizon and sub-conical, cup-shaped or glo- 

 bose in its entirety as are all the attached forms which we 

 are considering. To strengthen the walls of a round or- 

 ganism, the wall might be thickened by solid accumulation, 

 by vesicular formation or by perpendicular jjartitlons ar- 

 ranged at right angles to the supported wall. Snch me- 

 chanical supports are characteristic of attached Crelenter- 

 ates, Kndista\ Cirrhipeds. and some Vermes. As all sides 

 of the periphery of an attached organism are ecpially ex- 

 posed to food-supply, danger, etc., the organs, as tentacles, 

 nerve-centres and eyes, would gradnall}' tend to become 

 situated at all points on the periphery, or radially. It is 

 well known that the external parts of an animal are more 

 easily modified than the deeper-seated parts. It is also 

 known that the modification of deeper-seated parts may be 



