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 without impairing the general conclusions as to the effects of fixation. 



The RudistiB is a group having an enormously developed attached valve of higldy 

 irregular growth and a flattish free valve. The fi-ee valve may be of a form similar to 

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

 shown in Radiolites fleuriausa, PI. xxvi, figs. 11-12. Commonly, however, the free valve 

 of the RudistiB is radially plicated in the adult and diliers widely from any normal 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), alfecting the free as well as the fixed valve, instead of the latter 

 principally, as is the normal case. 



Dimya 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. aryentea by 

 Dall) which would connect it closely with Ostrea to which it has been compared by 

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

 conditions of fixation. 



The MuUeriacejG comprising the genera MuUeria, ^theria and Bartlettia is a divis- 

 ion of tUe Unionidaj, which in the adult condition resemble oysters. Mailer ui lohata,F ai:, 

 is so remarkably like an oyster that it has been called the fresh- Avater 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 commonly found in Ostrea vlrginiana. The shell as in 

 ^theria, also has a finely camerated or blistered structure. The young shell of MuUeria, 

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

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

 its symmetry and becoming irregular 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 genns it has 

 been named Ostrea and Ostracites. Hinnites cortesi, Defr., PI. xxvi, figs. 3-4, from 

 the Tertiaries, illustrates cleai'ly the characters found in the young and adult shell. We 

 will consider the two valves separately. In the young nealogic pei'iod the right valve, 

 fig. 3, is purely pecteniform. It has well developed ears, a deej:) byssal sinus, and an 

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

 marked period, as shown in the figure, the valve becomes attached to a foreign body by a 

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

 form immediately takes place. The shell grows irregularly, losing the evenness and in 

 considerable degree even the existence of the plications marking its nealogic period. 

 While the early nealogic shell was but slightly 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 osti-ealorm growth. The left five valve, Pi. xxvi, fig. 4, in the nealogic stage, 



