346 ROBERT TRACY JACKSON ON THE 



ill the form of the shell. Difterences in the anatomy and habits, however, exist, as dis- 

 cussed in previous pages. 



I have shown that the early, iiepionic shell growth of Peeten, PI. xxvur, figs. 9-10, has 

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

 lieve that the form of the latei- shell growth is also of [)hylogenetic significance. The 

 hinge line of Peeten irradians, in PI. xxviii, figs. 12-13, is i-elatively long as compared 

 with the condition found 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 Clilamys (Peeten) islaadica, PI. xxviii, fig. 11, Peeten 

 magellaniciis and P. disloccdiis; therefore it may be considered as typical of developing 

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

 under the genus Pterinopecten which agree closely with the foi-ni 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 characterize Pterinopeeten and 

 sex^arate it from the nearly allied genus Aviculopeeten. Pterinopecten is the least re- 

 moved from the Aviculidse, and is the simplest known form of the Pectinidse. The 

 young of Pterinopecten resembles the adult of Rliombopteria and the adult resembles 

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

 link in our phylogenetic series, and is placed as the first departure from the A\'iculid8e 

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



The palaeozoic genus Aviculopeeten, as amended by Hall, differs fi'oni Pterinopecten 

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

 developing shell of Peeten irradians, we find that the later groAvth, PI. xxviii, fig. 2, 

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

 inopecten. The same stage also differs from the adidt fig. 37, p. 333, as Aviculopeeten 

 differs from Peeten. This later stage is therefore traceable to Aviculopeeten as its an- 

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

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



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

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

 by t3'pical genera. It is further shown that in the development of a modern Peeten we 

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

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

 true Aviculidse, 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 Avieulidae and the Pectinidae. Still later a stage exists PI. id,., fig. 2, which is ref- 

 erable in its inherited form to A^aculopecten, and finally, the true Peeten 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 Peeten. We have, 

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

 group (see table, section xvi). 



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

 A\'iculidfB, it may be noted that in the Challenger rej^ort, Vol. xiii, part xxv. Professor 



