324 ROBERT TRACY JACKSON ON THE 



fixed Pelecypods that I know of, the normal condition of a deeper fixed valve is main- 

 tained. However, the relative size of the fixed and free valves is a factor, which may be 

 subject to variations withont impairing the general conclusions as to the effects of fixation. 



The Rudista? is a group having an enormously developed attached valve of highly 

 irregular growth and a flattish free valve. The free valve may be of a form simihir to 

 that found in unattached normal Pelecypods, while the fixed valve is most irregular as 

 shownm Badiolites Jleuricmsa,Fl.x:s:yi, figs. 11-12. Commonly, however, the free valve 

 of the Rudistte is radially plicated in the adult and differs widely from any noi'mal form 

 of Pelecypod shell. This extreme variation marked in both valves is accounted for as a 

 case in which the conditions of fixation have produced the extremes of modification 

 (see note, p. 322), aff'ecting the free as well as the fixed valve, instead of the latter 

 principally, as is the normal case. 



Diraya is a genus of rare and little-known Pelecypods. The shell is irregular os- 

 treaform and attached by the beak of the right valve which is deeper and larger than the 

 other. We fail to see any important characters of anatomy (described in D. argentea by 

 Dall) which would connect it closel}' Avith Ostrea to which it has been compared by 

 writers. From my view, whatever the affinities, its ostrean form of shell is due to the 

 conditions of fixation. 



The Mnlleriacese comprising the genera Mulleria, -i3Etheria and Bartlettia is a divis- 

 ion of the Unionidje, wliich in the adult condition resemble oysters. Mulleria lohata^Yew, 

 is so remarkabl}' like an oj^ster that it has been called the fresh-water oyster. In the 

 Monomyarian adult, PI. xxvi, fig. 5, the shell is rough and irregular, with a deep attached 

 and flattish free valve, and a specimen in the Museum of Comparative Zoology is indistin- 

 guishable in shape from forms commoidy found in Ostrea virginiana. The shell as in 

 ^theina, also has a finely camerated or blistered structure. The young shell of Mulleria, 

 PI. XXVI, figs. 6-7, is Anodon-shaped, equivalvular and dimyarian as described by au- 

 thors. In this genus we have the important feature of an Anodon-shaped shell, losing- 

 its symmeti'y and becoming iri'egular and ostreaform as soon as attached. 



Hinnites is an important genus in our considerations. In the young it is free and pec- 

 teniform, but in the adult is attached by the right valve, is irregular and more or less os- 

 treaform. So close is the likeness to an oyster that in the synonymy of the genus it has 

 been named Ostrea and Ostracites. Hinnites cortesi, Defr., PI. xxvi, figs. 3-1, from 

 the Tertiaries, illustrates clearly the characters found in the young and adult shell. We 

 will consider the tAVO valves separately. In the young nealogic pei-iod the right valve, 

 fig. 3, is purely pecteniform. It has well developed ears, a deep bj'ssal sinus, and an 

 evenly plicated shell which at this stage is nearly or quite equivalvular. At a definitely 

 marked period, as shown in tlie figure, the valve becomes attached to a foi'eign body by a 

 cementing fixation of the shell. With this cliange in condition, a most marked change in 

 form immediately takes place. The shell grows irregular)}^, losing the evenness and in 

 considerable dcgi-ee even the existence of the plications marking its nealogic period. 

 Wliile the early nealogic shell Avas but sliglilly concave and pecteniform the adult fixed 

 valve is deeply concave and highly ostreaform. The byssal notch, which is a marked 

 feature of the nealogic stage, is filled up and completely wiped out of existence by the 

 irregular ostreaform growth. The left live valve, PI. xxvi, fig. 1, in the nealogic stage, 



