318 ROBERT TRACY JACKSON ON THE 



flattened area on the lower valve. I have noted that Ostrea maj' in its eai-ly nepionic 

 period he attached, and yet not conform to the ohjeet, or hecome 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 ap])arently took ])lace in specimens of Exogyra which show no 

 flat area. In such thei-e is no proof that they Avere not fixed during the young stages, 

 hut merely that they dehisced hefore the shell conformed to the outline of the ohjeet of 

 support. Some individuals may he attached for a consideiahle extent, and a specimen in 

 the Amei'ican Museum in New York has an area of attachment measurins: 11 cm. in its 

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

 mens, as noted and figui-ed by 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 type, known as Excxjyra costata, and the smooth 

 type without costations known as E. ponder osa,^OQmi^v. The relations of these spec- 

 ies is about the same as that which exists betAveen Ostrea virf/iniana. Lister, and O. 

 horealis, Lam., which Vcrrill unites under the former species, as he says the}' may easily 

 be connected by all sorts of intei-mediate forms. 



In the Ostreadse there is a stiiking peculiarity of the adult which has not been con- 

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

 heing concave and the other flat; hut they are not only unequcd, they are very dissimilar, 

 as different as if they helonyed to distinct sjjecies in what would he considered tyjjical 

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

 ]-ect cemented fixation acting upon a Pelecypod shell, and is of great value in consider- 

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

 ing pages. 



As examples amongst the Ostreadse, showing dissimilarity of the tAvo valves, Ave may 

 take Ostrea edidis of Europe, O. compressirostris, h-om the Eocene of this countiy, and 

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

 and the u|)per smooth, and i-egular-groAving, without plications. The examples could be 

 multiplied extensively, but these sutRce to give my meaning. In consulting Coquand's 

 finely illustrated monograph of Cretaceous oystci'S, examples of this difierence of the tAVO 

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

 of inherited or acqimxd characteristics, finding very different expression in the two valves 

 of a group heJonging to a class tyjncally eqitivalvidar. As might be exjiected, A'ariations 

 fi'om the rule are occasional as in Ostrea marshii, 0. larva, etc., Avhere the tAvo valves are 

 more closely alike. Dissimilarity is ncA'ertheless the rule. The u])per valve may bear 

 characteristic features not found in the lower A'alve. As an example Ave may mention 

 Gryjjhcea vesicularis of the Ci'etaceous, in Avhich the upper valve in both European and 

 American s])ecimens is marked by radiating depressed lines. Similar radiating lines 

 may be found in Ct. dilatata fi-om the Jurassic, and indications of them have been ob- 

 sei'A'ed in other si)ecies. These lines are a feature which Avoidd probably be of value in 

 tracing the serial connections of the several species possessing them. 



The ditferences in the tAvo valves may be observed in species of Spondyli, especially 

 those in which the right, attached valve is very ostreafoi-m. Avhile the ii-ee, left valve is 



