PHYLOGENY OF THE PELECYPODA. 



323 



proof of this lact and then special cases will be considered which show the relation of 

 cause and effect. 



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

 flatter left and a highly developed camei'ated sti'uctnre. The growth is iri-egular, 

 spinous, uneven, and the lower valve is often highly ostreaform as shown in SjJondylus 

 Mpjmritarum, PI. xxvi, fig. 1. The group is called spiny oysters from the typical spiny 

 growth and ostrean form. In the Challenger Eeport, a species is described as Spondylus 

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

 features of comparison being marked in both valves. Plicatula, 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 Ostreadae, becomes attached at the close of the prodissoconch 

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

 period of development. 



The ChamidfB as a grou]) are characterized by ostrean features, though departni-es 

 in degi-ee rather than in kind are seen in several genera. The genus Chama has typi- 

 cally a deep, iiregulai-, thick, attached valve and fiatter, thinner, free valve. A Terti- 

 ary species is named Chama grypheata, on account of its close likeness to the genus 

 Gryphfea. Chama constitutes one of the best proofs that the condition of fixation 

 causes the form of the valves, because some species are attached inditfei-ently by the 

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

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

 right valve, is almost indistinguishable in external chai*acteristics from many forms of 

 Exogyra. The aberrant foi-ms of the Chamida?, as Caprinula, fig. 8, Monopleura, fig. 9, 

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

 sidered as exti'emes of variation due to fixation. The attached valves are deep and 

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

 valves, though frequently modified, are yet not dilFerent from what is often met with 

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

 fied valves and extreme young 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 i)rij(liiceil from •vvitlumt inwards fexaniple : 

 ilevelopment of internal from external skeletons in fishes), 

 or ce»tri]ietalhj as I would call it. To retnrn, if we have 

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

 ranged on the periphery (a very common case) , theu fur- 

 ther modification or development of such organs or parts 

 would tend to be produced cenCripcUdly 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. Pa,ssing from Photohydra 

 to Uydra, to the liydroid stage of Aurelia, then to the free 

 medusoid stage of Aurelia and other Discophores, we find 

 progressively a more and more perfect radial symmetry 

 built up ceutripetally. Passing to free swimming Hydro- 

 zoa, as the Ctenophores, radial symmetry is eradicated, 

 and an almost complete bilateral symmetry adopted by the 



adult. A similar line of comparison may l)e made in the 

 development of corals. Cirrhipeds in the shell show a 

 very complete radial symmetry especially seen in Coronula. 



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 radiality also is induced and i)rogressively built up 

 by Ihe conditions of permanent fixation with its attendant 

 intluences of environment. 



Many animals are attached by more or less fleshy i)roc- 

 esses from the body, as Braehiopods by a peduncle and 

 Pelecypods by a byssus, but these do not adopt a round 

 form, as do those groups which are attached by a more 

 permanent calcareous fixation. 



