PHYLOGENY OF THE PELECYPODA. 387 



intermedins, Hall, P. vertumnus, Hall, and P. conspectus, Hall. Pterinopecten is nat- 

 urally derived from Ehombopteria by the introduction of slight ears on the hinge line. 

 It is the simplest known form of the Pectenidse and is ti'aceable in the early stages of 

 develoj^ment of Aviculopecten and Pecten; therefore, I shall consider Pterinopecten as 

 directly descended from Rhombopteria and as the base of the Pectinidae from which all 

 later forms were derived. (Further discussion of Pterinopecten, see p. 346.) 



Aviculopecten (McCoy), as amended by Hall, includes forms with a hinge usually 

 shorter than the transverse diameter of the shell and with both ears well defined. A carti- 

 lage pit is described by Hall in several species of this genus, also in Lyrioi^ecten and other 

 allies. A deep byssal sinus occurs in the right valve of many species of Aviculopecten 

 and diverging furrows exist on the hinge line in some species similar to those found in 

 certain recent Pectens as P. maxlmus. A finely preserved specimen of Avicnlojxcten 

 fasciculatus, Hall, shows that when yoimg (3 mm. in height) the shell was rhomboidal 

 and in its general contour, resembled Rhombopteria. During later growth a long hinge 

 line and moderately developed ears develop, making the shell at this stage comparable to 

 Pterinopecten ; later still, the characters of a shorter hinge line and pronoitnced ears give 

 the shell the characters of the genus Aviculopecten. Similar observations have been 

 made on the development of Aviculopecten p)rlnceps. Hall, and a few other species. Avi- 

 culopecten, when young, resembles Pteiinopecten and when adult resembles the young 

 of Pecten, PI. xxvin, fig. 2. It is, therefore, unquestionably intermediate between these 

 two types. (Further discussion of Aviculopecten, see p. 346.) 



From Aviculopecten we pass to Pecten in the direct line of ascent. Pecten has a rel- 

 atively short hinge line, both ears well defined, but the ventral (anterior) a little larger; 

 either a slight or no byssal sinus exists in the ear of the right valve. The ligament and 

 cartilage are as in Aviculoj^ecten. In the development of Pecten irradians and other 

 species (sections ix-x) we have shown a remarkable series of phylogenetic characters 

 which may be briefly alluded to. The prodissoconch, ^j, of Pecten is dimyarian, PI. 

 xxvir, figs. 8-9, PI. xxviir, figs. 9-10, and is traced to the Nuculoid radical of the whole 

 gi'oup (see table). The first nepionic stage, PI. xxviii, fig. 9, is rhomboidal, oblique, and 

 is traced to Rhombopteria; the later nepionic stage, PI. xx\aii, figs. 12-13 has a long 

 hinge line, and ears which are simple expansinns of the hinge margin; these features 

 and the deep byssal sinus are traceable to Pterinopecten. The nealogic stage of Pec- 

 ten, PI. xxvni, fig. 2, has a relatively shorter hinge and well developed ears, and this 

 stage is traceable to Avicidopecten. Only when the individual has advanced consid- 

 erably in age are the full characters of the adult, fig. 37, p. 333, acquired. The first 

 appearance of Pecten geologically is doubtful as the limitations of the genus among pa- 

 laeozoic fossils has not been fully worked out. All of Professor Hall's Devonian Pectin- 

 idai are, however, referable to other genera and we will provisionally consider Pecten as 

 dating from the Carboniferous. From Pecten as a base many genei-a have sprung as 

 will be presently consideivd. 



Lyriopecten (Hall) differs from Aviculopecten in possessing a short hinge and a small 

 anterior ear. It also differs from Pecten in this last character and it is considered as a 

 side branch from Aviculopecten. In support of this view we find that a specimen of 

 Jjijriopecteii orhiculatus Hall, which had a shoi't hinge in the adult, in the young (as ascer- 



