PHYLOGENY OF THE PELECYPODA. 325 



is even and regularly pectenifoi'iii like the right valve. AYheii the right valve becomes 

 attached, the left valve affected somewhat by the same conditions loses much of the 

 regularity and evenness of marking characterizing its nealogic period, but it never be- 

 comes as irregular in growth and ostreaform as does the right (attached) valve. Hin- 

 nites is marked therefore by a byssated pecteuiform stage followed by an attached os- 

 treaform stage. The transitions described in Tlianites cortesii, I have also observed 

 in the living species H. gigantea, Gray, and H. sinuosus, Gmelin. 



Ostrea we have shown becomes attached at the close of the prodissoconch period, and 

 the same is true of Plicatula (see section xi) ; as soon as attached both these genera, as 

 in Hinnites and MuUeria, assume the irregular ostrean form of shell, losing entirely the 

 symmetry in the valves of the early nnattached period. 



Pernostrea, a Jurassic genns, is of interest in these studies. According to Stoliezka 

 the shell is "rounded or oval, solid, more or less tumid, inequivalve, the left valve being 

 in adult specimens attached; structure lamellar, resembling that of Perna . . " 

 Again Stoliezka says: "This genus forms a connecting link between Melina (Perna) 

 and Ostrea, differing from the foi'mer especially by its sessile habitat, absence of a bys- 

 sal sinus and strongly excavated muscular scar; fi'om the latter by the presence of sep- 

 arate ligamental grooves. Externally Pernostrea is barely distinguishable from Ostrea." 

 From our point of view, the ostrean form is due to the condition of fixation. Therefore 

 Pei'uostrea is like Ostrea because it is cemented to foreign bodies. I have shown that 

 in Hinnites, PI. xxvr, figs. 3-4, the byssal notch disappears as soon as the shell becomes 

 soldered to an object of fixation, as the byssal attachment at that period is abandoned 

 and the shell assumes an irregular ostrean growth. Therefore the disapjiearance of a 

 byssal notch in Pernostrea is another character fairly atti'ibutable to the condition of 

 cemented fixation. In the species of Perna in PI. xxvr, figs. 16-18, we find that from the 

 typical form of this genus having a long hinge area and many cartilage pits, fig. 16, a se- 

 ries may be traced to a form with few cartilage pits and wing-like productions of the 

 shell, fig. 18. If we should carry this reduction of the cartilage pits still further till only 

 one remained while still retaining the wing-like productions of the shell we should have a 

 form similar to the nearly allied genus Malleus. Still more to the point is the fact that 

 in studying young Perna I have found that at first only one cartilage pit exists, which is 

 triangular in outline and oblique in position as in Meleagrina, Malleus and Ostrea. It 

 seems that there is no difficulty in a single cartilage pit being derived from many car- 

 tilage pits; so, that other points being considei-ed, Pernostrea may very properly be con- 

 sidered as intermediate in form between Perna and Ostrea. As Pernostrea is known 

 only in the Jurassic, and true oysters are known from older formations, it is best to con- 

 sider Pernostrea not as in the line of ancestry of the Ostreadje, but as a separate branch 

 from Perna parallel to the bi'anch on which the Ostreadai evolved as discussed at the 

 close of this section and in section xvi. 



In looking back to the earliest known oystei'S, we find that they are very typical, in- 

 equivalvular ostrean forms. M. Barrande's Prieo.^trea hohemica, Barr., a form which 

 seems i-eferable to the oysters, is based on isolated valves attached by the umbo of appar- 

 ently the left valve. His PL 111, fig. 2, is an upper free valve and bears at the extreme 

 tip a rounded, veiy marked early stage which resembles the ostrean prodissoconcli as 



MEMOIKS BOSTON SOC. NAT. HIST., VOL. IV. 4t 



