80 Annals of the South African Museum. 



the features of the nepionic and neanic stages of G-ryphcea as illus- 

 trated, for instance, in the admirable descriptions and figures given 

 by Hill and Vaughari of members of the genus occurring in the Lower 

 Cretaceous strata of Texas/ 1 ' When we see how closely the manner 

 of growth in the adult stage of E. imbricata often simulates that of 

 a true Gryphaa with simple ostrean ancestry, it must be admitted 

 that the parallelism is very striking. 



It would be difficult to indicate within narrow limits the true 

 relationship of Exogyra imbricata, but it is evident that its nearest 

 morphic counterparts are essentially characteristic of Lower Creta- 

 ceous rocks. Thus, a very close resemblance exists between this 

 form and some of the shells classed by Leymerie under the name 

 Exogyra sinuata and E. subsinuata. Leymerie first regarded all 

 these as varieties of Exogyra sinuata,^ and included forms to which 

 numerous specific names had previously been applied. He subse- 

 quently separated E. subsinuata as a species distinct from E. sinuata, 

 and used several varietal designations of both. Eegarding the 

 value of the nomenclature employed by this author or by CoquandJ 

 in dealing with the forms comprised within this perplexing group, 

 I am at present unable to attempt the critical and detailed study 

 which alone would warrant the expression of any judgment ; in like 

 manner, the varying use of the name Exogyra couloni by different 

 authors has given rise to such a degree of confusion that, in insti- 

 tuting comparisons, it must suffice to make reference to actual 

 figures or descriptions, leaving aside for the time being all question 

 of the value or correctness of the specific names attached to certain 

 of these. 



Krauss rightly recognised the broad relationships of E. imbricata 

 and believed it to be nearly related to " Gryphaa couloni Defr.," 

 from which, however, he found it to differ in the following points : 

 "It is larger, more thick- shelled, longer and narrower ; has a keel 

 narrower at the end (though always rounded), more concave laminae 

 lying much higher one upon the other, and a stronger umbo bent 

 further over the opercular valve, than in Gryphaa couloni Defr." 

 Some of the shells figured by d'Orbigny as Ostrea couloni show 

 no slight resemblance to selected individuals of E. imbricata, but are 

 narrower than the generality of the Uitenhage specimens, and the 

 imbricating lamellae of the larger valve are more prominent and 

 irregular. The broad, plicated specimens figured by d'Orbigny 

 under the same name on plate 466 of his work cannot be brought 



* Hill and Vaughan (1). f Leymerie (1). 



} Coquand (2). d'Orbigny (3), pi. 467, figs. 1-3 (1848). 



