318 ROBERT TRACY JACKSON ON THE 



flattened area on the lower valve. I have noted that Ostrea may hi its early nepionic 

 period be attached, and yet not conform to the dbjeet, or become flattened at all, sothatif 

 it could have separated at the close of this period, the adult would show no sign of early 

 fixation. This is what apparently took place in specimens of Exogyra which show no 

 flat area. In such there is no proof that they were not fixed during the yoiing stages, 

 but merely that they dehisced before the shell confoi-med to the outline of the object of 

 support. Some individuals may be attached for a considei'able extent, and a specimen in 

 the American Museum in IS^ew York has an area of attachment measuring 11 cm. in its 

 greatest length. This species is also very variable in the degree of costation. Aged speci- 

 mens, as noted and figured l)y Whitfield, often lose the ridges and become quite smooth. 

 I have seen many specimens in which the lower valve was first smooth, then costate, and 

 later smooth again, or the reverse of this order. In a large collection one may find every 

 gi'adation between the highly costate tyjie, known as Exogyra cosiafa, and the smooth 

 ty2)e without costations known as Ji*. ^jo)KZero.«/, lioemer. The relations of these spec- 

 ies is about the same as that which exists between Ostrea virginiana, Lister, and O. 

 horealis, Lam., which Verrill unites under the former species, as he says they may easily 

 be connected by all sorts of intei'uiediate forms. 



In the Ostreadaj there is a striking peculiarity of the adult which has not been con- 

 sidered in its full significance, so far as I am aware. The two valves are unequal, one 

 heing concave and the other flat; htit they are not only unequal, they are very dissimilar, 

 as different as if they ielonged to distinct species in what ivoidd be considered typical 

 forms. This character of the shell is due, I believe, to the mechanical conditions of di- 

 lect cemented fixation acting iipon a Pelecypod shell, and is of great value in consider- 

 ing the systematic relations of attached groups as I shall attemjit to show in the follow- 

 ing pages. 



As examples amongst the Ostreadfe, showing dissimilarity of the two valves, we may 

 take Ostrea edulls of Europe, O. compressirostris, from the Eocene of this country, and 

 Exogyra costata, from the Cretaceous. In these species the loAver valve is plicated, 

 and the upper smooth, and regular-growing, without plications. The examples could be 

 multiplied extensivel}', but these suffice to give my meaning. In consulting Coquand's 

 finely illustrated monograph of Cretaceous oysters, examples of this diffei'cnce of the two 

 valves Avill be found in many species. This is a highly interesting feature, as it is a case 

 of inherited or acquired characteristics, flnding very diff'erent expression in the two valves 

 of a group belonging to a class tyjnccdly equivalvulur. As might be expected, vai-iations 

 from the ru'e are occasional as in Ostrea marshii, 0. larva, etc., where the two valves are 

 more closely alike. Dissimilarity is nevertheless the rule. The upper valve may bear 

 characteristic features not found in the lower valve. As an example we may mention 

 Oryphcea vesicularis of the Cretaceous, in which the upper valve in both European and 

 American specimens is marked by radiating depressed lines. Similar ladiating lines 

 may be found in G. dilatata from the Jurassic, and indications of them have been ob- 

 sei'ved in other sjiecies. These lines are a feature which would probably be of value in 

 tracing the sei'ial connections of the several species possessing them. 



The differences in the tAVO valves may be observed in species of Spondyli, especially 

 those in which the right, attachtd valve is very ostreaform, while the free, left valve is 



