PHYLOGENY OF THE PELECYPODA. 319 



less modified, as shown in Spondiilus lupparltaruiu, d'Orlj., PI. xxvi, fig. 1. We would 

 also refei- the reader to M. d'Oi'biguy's plates 450 to 461 of Ci'etaceous Spoiidyli, es- 

 pecial attention being called to his figures of 8. histrix, Groldf., in which the attached 

 valve is rough and o-streaform, whereas the free valve is evenly jilicated and smoother. 

 Striking differences in the features of the two valves may be seen in the Rudistae as 

 shown in BadloUtes fleiirlausa, d'Orb., PI. xsvi, figs. 11-12, in which the lower at- 

 tached valve is irregularly cornucopia-shaped, whereas the upper free valve is compara- 

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

 species of Ilippurites studied resemble an Exogyra (compare the figures cited with the 

 adult and young of Caprotina semistrlata, PI. xsvi, figs. 10 and 20). Because the young 

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

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

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

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

 habit common to both groups, and not as exjiressing 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. 



Al)errant forms of the Cliamidae are shells which show in a liigh degree differences 

 in the attached and free valves. Capr inula holssiji, d'Orb., PI. xxvi, fig. 8, from the 

 Cretaceous of Europe is a remarkable form; the attached valve is deeply conical with 

 perpendicular rugae ,whereas the free valve is spirally twisted, without plications and re- 

 sembles a typical Diceras, Caprotina semlstriata, d'Orb., PI. xxvi, fig. 10, is aiiother 

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

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

 spirally twisted tip as in Ridiolites, PI. xxvi, fig. 12. The free valve is of a simple con- 

 cave form, not at all aberrant and not presenting the rugae seen in tha attached valve. 

 A very young specimen of Caprotina semistrlata, PI. xxve, fig. 20, was found attached 

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

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

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

 free valve convex showing little twisting as in Exogyi-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 I'ender the form and its relations to the adult 

 clearer. That they are fairly represented will be evident upon a comparison with similarly 

 twisted shells where they arepreserved. As the attached valve in Caprotina, Pi. xxvi, 

 fig. 20, gi'ows from its early twisted condition to the deeply conical condition found in 

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

 more and more hoiizontal position until in the adult they are nearly or quite hoi-izontal 

 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 deejily 

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

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

 xxvi, fig. 9. In Dr. White's paper (74) the same feature and the modification of at- 

 tached shells are veiy well shown in man^^ species. 



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



