PHYLOGENY OF THE PELECYPODA. 



323 



proof of this fact and then special cases will be considered Avhich show the relation of 

 cause and effect. 



The genus Spondylus is attached by the right valve. It has a concave right valve, 

 flatter left and a highly developed camerated structure. The growth is irregular, 

 si)inous, uneven, and the lower valve is often highly ostreafoi'm as shown in Spoiidylus 

 hijipuritannn, PI. xxvi, fig. 1. The group is called spiny oysters fioni the ty])ical spiny 

 gi-owth and ostrean form. In the Challengei- Eeport, a species is described as Spondylus 

 ostreoides. Smith, so named, on account of the close siniilaiity to a small oyster, the 

 features of comparison being marked in both valves. Plieatula, a genus nearly related 

 to Spondylus and attached by the i-ight valve, is also highly ostreaform. This genus, 

 as is the case with the Ostieadae, becomes attached at the close of the prodissoconch 

 stage, as shown in PI. xxvil, fig. 5, and the ostiean characters are assumed at that 

 l)eiiod of development. 



The Chamidre as a group are characterized by ostrean features, though departures 

 in degi-ee rather than in kind ai'c seen in several genera. The genns Chama has typi- 

 cally a deep, irregulai-, thick, attached valve and flatter, thinnei', free valve. A Terti- 

 ar}' sjiecies is named Chama grypheata, on account of its close likeness to the genus 

 Gi-yplnea. Chama constitutes one of the best proofs that the condition of fixation 

 causes the form of the valves, because some species are attached indifferently by the 

 right or left valve, and in both cases the resulting form is the same, a deep attached 

 and a flatter free valve. Chamostrea alhida, PL xxvr, fig. 2, which is attached by the 

 right valve, is almost indistinguishable in external characteristics from many forms of 

 Exogyra. The aberrant fonns of the Chamidfe, as Caprinula, fig. 8, Monopleura, fig. 9, 

 and Caprotina, fig. 10, PI. xxvr, also similar allied genera, are forms which may be con- 

 sidered as extremes of variation due to fixation. The attached valves are deep and 

 mostly cornncopia-shaped, often highly irregular and asymmetrical, whereas the free 

 valves, though frequently modified, are yet not different fi'om what is often met with 

 in genera of unattached Pelecypods. In these peculiar genera it is the free less modi- 

 fied valves and extreme j^omig to which we must turn in studying the relationships of 

 the groups. In occasional species the attached valve is not deeper than the free valve, 

 which is the larger and deeper of the two. Such forms are exceptional, as in all other 



progressively produced from without inwards Cexample : 

 development of internal from external skeletons in fishes), 

 or centripetaVy as I would call it. To return, if we have 

 an attached animal that is round, with some organs ar- 

 ranged on the peripherj' (a very common case), then fur- 

 ther mcdiflcation or development of such organs or parts 

 would tend to be produced ci-nlripeUtUy or radially, for the 

 centripetal variation of a round form equally exposed to 

 modification on all sides, would, as a mechanical necessity, 

 be radial variation. In the Hydrozoa there is strong proof 

 of the correctness of this view. Passing from Thotohydra 

 to Hydra, to the hydroid stage of Aurelia, then to the free 

 medusoid stage of Aurelia and other Discophores, we find 

 progressively a more and more' perfect radial symmetry 

 bnilt up ceutripetally. Passing to free swimming Ilydro- 

 7.oa, as the C'tcnophnres. radial syninietrv is eradicated, 

 and an almost complete bilateral symmetry adopted by the 



adult. A similar line of comparison may be made in the 

 development of corals. Cirrhipeds in the shell show a 

 very complete radial symmetry especially seen in Coroinila. 



Many permanently attached forms of bilateral classes of 

 animals assume a considerable degree of radial symmetry 

 and, conversely, many free locomotive genera of radiated 

 animals assume a considerable degree of bilateral symme- 

 try. As bilaterality is a feature induced and progressively 

 built up by the conditions of free locomotion, it is believed 

 that radlality also is induced and progressively built up 

 by the conditions of permanent fixation with its attendant 

 iufluenees of environment. 



Many animals are attached by more or less Iteshy proc- 

 esses from the body, as Bracliiopods by a peduncle and 

 Pelecypods by a byssus, but tlicse do not ado|)t a round 

 form, as do those groups wliich are attaelnil by a more 

 permanent calcareous fixation. 



