PHYLOGENY OF THP: PELECYPODA. 319 



less modified, as shown in Spondylus hippiiritarum, d'Orb., PI. xxvr, fig. 1. We would 

 also refer the reader to M. d'Orbiguy's plates 450 to 461 of Cretaceous Spondyli, es- 

 pecial atteution being called to his figures of >S'. liistrix, Goldf., in which the attached 

 valve is rough and ostreaforni, whereas the free valve is evenly ])licated and smoother. 

 Striking difi'ei'ences in the features of the two valves may be seen in the Rudistae as 

 shown in RadloUtes Jleariaasa, d'Orb., PI. xxvi, figs. 11-12, in which the lower at- 

 tached valve is ii'i-egularly cornucopia-shaped, whereas the upper free valve is compara- 

 tively regular growing, flattish and plicated. The young of this species and several 

 species of Hippurites studied resemble an Etogyra (compare the figure^ cited witii the 

 adult and young of Caprotina semislrlata, PI. xxvi, figs. 10 and 20). Because the young 

 of these aberrant forms resemble au Exogyra, I do not wish to imply any genetic connec- 

 tion with that group. I state later, p. 322, the conviction that the ostrean form is in- 

 duced by the condition of fixation, and the similarity to an Exogyra in the young of the 

 Rudista; and Chamidaeis believed to be due to the similar conditions of a fixed cemented 

 habit common to both gi-oups, and not as expressing any affinities. This is a point which 

 must be understood, as in this paper high importance is attached to the form of young 

 shells in tracing genetic relations. 



Aberrant forms of the Chamidae are shells which show in a high degree diffjrehces 

 in the attached and free valves. Caprinida hoissyi, d'Orb., PI. xxvi, fig. 8, from the 

 Cretaceous of Eui'ope is a remarkable foi-m; the attached valve is deeply conical with 

 perpendicular rugse ,whereas the free valve is spn-ally twisted, without plications and re- 

 seml)les a typical Diceras, Caprotina semistriata, d'Orb., PI. xxvr, fig. 10, is another 

 form not less remarkable in the striking dissimilarityof the two valves. In this species the 

 attached valve is arcuately conical with perpendicular ruga? and showing at its base a 

 spirally twisted tip as in R idiolites, PI. xxvi, fig. 12. Tlie free valve is of a simple con- 

 cave foi'm, not at all aberrant and not presenting the rugie seen in ths attached valve. 

 A very young specimen of Cajjrotina semistriata, PL xxvi, fig. 20, was found attached 

 to the base of a colony of this species in a specimen in the Museum of Comparative 

 Zoology. This young Caprotina resembles an Exogyra, taking for comparison a young 

 J^. costata or an adult E. spiralis, Goldf. The attached valve is highly twisted and the 

 free valve convex showing little twisting as in E.xogyi'as. The specimen was too much 

 ■worn to show lines of growth, but in the figure they are inserted where I think they 

 would appear if not eroded, in order to render the form and its relations to the adult 

 clearer. That they are fairly represented will be evident u|)on a comparison with similarly 

 twisted shells where they arepreserved. As the attached valve in Caprotina, PI. xxvr, 

 fig. 20, grows from its eaily twisted condition to the deeply conical condition found in 

 the adult, PI. xxvr, fig. 10, the lines of growth gradually and progressively assume a 

 moi'c and more horizontal position until in the adult they are nearly or quite horizontal 

 and the attached valve has lost the twisted condition characteristic of the younger stages. 

 The outgrowth from an early twisted condition of the attached valve to an adult deeply 

 conical condition, as just described, doubtless took place in the development of Caprinida 

 hoissyi, PI. XXVI, fig. 8, and the change may be seen in Monopleura marcida. White, PI. 

 xxvr, fig. 9. In Dr. White's paper (71) the same feature and the modification of at- 

 tached shells are very well shown in many species. 



Our examples of differences iu the two valves amongst attached Pelecypods might 



