38G ROBERT TRACY JACKSON ON THE 



and in this area tlie lines of growth pass directl}' across from one valve to the other 

 without any indication of the hinge line. The naci-eons tissue is highly reduced in ex- 

 tent. The shell has become in a physiological sense a flattened tulnilar univalve which 

 may be compared in a measure in its resultant form to an Aspergillum {vide note 3, p. 

 376). In the young of Atrina the two valves are perfectly separable and have not yet 

 acquired the twisted al)normal features characteristic of the adult. Atrina sliould be con- 

 sidered a very retrograde genus as it is weak and abnormal to a high degree. 



Trichites, another genus derived from Pinna, is peculiar for its excessive development 

 of prismatic cellulai- tissue which may reach an inch or more in thickness. It is an ex- 

 treme modification of the Pinnida? of highly ii-regular growth and without hinge teeth. 

 The great development of prismatic cellular tissue in the Pinnidae disproves the state- 

 ment commonly made in text-books that this structure is only formed by the edge of the 

 mantle. The formation of prismatic tissue commonly is limited to that portion of the 

 shell built by the edge of the mantle, as in Avieula, Ostrea, Anodon, etc., the deeper-seated 

 portions of the mantle in such cases building naci'eous or sub-nacreous tissue. In Pinna, 

 Atrina, Trichites and Malleus, however, the deeper seated parts of the mantle as well as 

 distal edges must build prismatic tissue or this structure would not attain the thickness that 

 it does, and further we may actually see in fresh specimens that the nacreous structure, as 

 in Atrina and Malleus, is limited to a very small area near the umbonal portion of the shell. 



Pterinea is closely related to Aviculoid tj'pes and jQi is not in the ascendent line of 

 Avieula proper. The shell is nasute anteriorly and the wing may or may not be pro- 

 duced posteriorly. The ligamental area of the valves is striated longitudinally (compare 

 with Meleagrina) ; but no cartilage pit is differentiated, although this is characteristic 

 (either as a single or several pits) of Avieula and all its various descendants as represent- 

 ed in the accompanying table. The young of Pterinea as shown by Goldfuss' in P. IcBvis 

 is rhombic in form and bears a very close resemblance in contour to Rhombopteria, fig. 51, 

 p. 380. Pterinea is considei-ed as a side branch, probably derived from Rhombopteria. 



Aetinopteria (Hall) and Ptychopteria (Hall) are genera described by Professor Hall as 

 differing from Pterinea in the absence of a strong striated ligamental area and in the ab- 

 sence of cardinal and lateral teeth. Following his figures and descriptions, it is seen that 

 Aetinopteria more nearly approaches to the characters of Pterinea than does Ptychop- 

 teria; therefore, in our table they are arranged in that serial order. Both genera appear 

 to be on a side branch from Pterinea; not in a direct line of ascent because they are char- 

 acterized by a loss, not an accentuation or modification of Pterinean features. 



Pterinopecten (Hall), a Devonian genus of the Aviculopecten group, is characterized by 

 a long hinge line, with ears not well defined, being simple expansions or extensions of the 

 hinge line. The various species are frequently described by Hall as sub-rhomboidal, espec- 

 ially when yoiing, and he mentions that some sjiecies resemble the young of Aviculopecten 

 and Lyriopecten. Pterinopecten dignatus, Hall, P. conspectus, Hall, and similar species 

 bear a close resemblance to the young of Pecten irradians, PI. xxviii, figs. 12-13. They 

 resemble the figures cited in both valves as the right valve has a deep byssal sinus. The 

 young of Pterinopecten is similar to the adult of Rhombopteria as I have observed in P. 



' I't'trefacta Germauife, 1*1. cxix, fig. la. 



