346 ROBERT TRACY JACKSON ON THE 



in the form of the shell. Differences in the amitomy and habits, however, exist, as dis- 

 cussed in previous pages. 



I have shown that the early, nepionic shell growth of Pecten, PI. xxviir, figs. 9-10, has 

 a form referable in origin to the adults of ancient members of the Aviculidtc, and I be- 

 lieve that the form of the later shell growth is also of phylogcnetic significance. The 

 hinge line of Pecten irradlans, in PI. xxviii, figs. 12-13, is relatively long as compared 

 with the condition foiind in the adult of this or other species of Pectens, and correlative- 

 ly the ears are very slightly developed as compared with the adult. The same condition 

 of relative great length of hinge line and want of development of the ears is seen in sim- 

 ilar stages of development of Chlamys (Pecten) islandica, PI. xxnn, fig. 14, Pecten 

 magellanicus and P. dislocatus; therefoi-e it may be considered as typical of develoj)ing 

 Pectens. In Hall's Palaeontology, Vol. v. Part i, we find forms of Devonian Pectinidae 

 under the genus Pterinopecten which agree closely with the form of shells here figured 

 in developing modern Pectens. The agreement is in the relatively great length of the 

 hinge line and slight production of ears, which features chai-acterize Pterinopecten and 

 separate it from the nearly allied genus Aviculopecten. Pterinopecten is the least re- 

 moved from the Aviculidje, and is the simplest Iviiown form of the Pectinidae. The 

 young of Pterinopecten resembles the adult of Rhombopteria and the adult resembles 

 the young of Aviculopecten and Pecten (see section xvi). It forms, therefore, a strong 

 link in our phylogcnetic series, and is placed as the first departure from the Aviculidae 

 in the Pecten line of variation (see discussion and genealogical table, section xvi) . 



The palaeozoic genus Aviculopecten, as amended by Hall, differs from Pterinopecten 

 in a greater production of the ears and in a relatively shorter hinge line. Turning to the 

 developing shell of Pecten irradians, we find that the later growth, PI. xxviii, fig. 2, 

 differs from the earlier growth, PI. id., figs. 12-13, as Aviculopecten differs from Pter- 

 inopecten. The same stage also differs from the adult fig. 37, p. 383, as Aviculopecten 

 differs from Pecten. This later stage is therefore traceable to Aviculopecten as its an- 

 cestral representative, and also the position of Aviculopecten is clearly indicated in its 

 relation to other genera (see discussion and table, section xvi). 



The prodissoconch of Pecten as stated (p. 342) is referable in origin to the Nuculoid 

 radical, represented in the development of the Aviculidae and their allies as illustrated 

 by typical genera. It is further shoAvn that in the development of a modern Pecten we 

 find in the first stages of dissoconch growth a form of shell, PI. xxviii, fig. 9, iDresenting 

 characters which make it referable in ancestral origin to Rhombopteria a member of the 

 true Aviculidae, later succeeded by a growth, PI. id., fig. 12, bearing marked features 

 referable in origin to an ancestral genus Pterinopecten, which is ti'ansitional between 

 the Aviculidjia and the Peetinida?. Still later a stage exists PI. id., fig. 2, -which is ref- 

 erable in its inherited f )rm to Aviculopecten, and finally, the true Pecten features char- 

 acteristic of the adult, fig. 37, p. 333, are established. The geological sequence of 

 these several groups is in the order indicated by the development of Pecten. We have, 

 therefore, a clear case of the ontogeny of an individual illustrating the phylogeny of a 

 group (see table, section xvi). 



As further substantiation of my position that the Pectinidae were evolved from the 

 Aviculidae, it may be noted that in the Challenger report, Vol. xin, part xxv, Professor 



