78 Annals of the South African Museum. 



1867. Ostrea imbricata E. Tate, Quart. Journ. Geol. Soc., vol.xxiii., 



p. 154. 



1904. Pycnodonta imbricata H. Douvill6, Bull. Soc. Geol. France, 

 ser. 4, tome iv., p. 215. 



Occurrence. This form is widely and abundantly distributed in 

 the Marine Beds of the Zwartkop's, Sunday's, and Coega Eiver 

 valleys. It was met with at almost all the exposures in the 

 Zwartkop's valley visited by Messrs. Eogers and Schwarz. Ex- 

 amples sent to me are from the railway cutting between milestones 

 24J-24J on the railway from Uitenhage to Graaff-Eeinet (312), 

 and others were collected by Miss M. Wilman at Coega. Speci- 

 mens sent from the collection of the South African Museum are 

 from the Sunday's Eiver (300, 301, 302, 303). 



Remarks. A striking character of the late adult and senile 

 condition of E. imbricata consists in the prodigal manner in which 

 shell substance is added at the margins of the valves, particularly 

 noticeable in the smaller (right) valve in the neighbourhood of the 

 hinge. Here, with its margin consisting of many coarse, imbri- 

 cating laminae, this valve may attain a thickness exceeding 30 mm. 

 The ligament pit is seen to be strongly curved near the apex, but 

 straight in the portion representing the later growth-stages. 



This abundant and characteristic shell has given rise to some 

 differences of opinion concerning its generic position within the 

 Ostreidae, which, however, may be sufficiently accounted for by the 

 great variability of form exhibited by different individuals, and by 

 the fact that the shell presents features which are recognised as 

 leading characteristics of both Exogyra and Gryphcea. For this 

 reason Tate used the name Ostrea in the broad sense, stating his 

 opinion that the species in question " affords a good illustration of 

 the mere sectional value (which cannot at all times be employed) of 

 the groups Exogyra and Gryph&a." In many instances, it must 

 be admitted, apparent passage forms seem to render difficult the 

 definite separation of the three genera, so easily distinguishable 

 when represented in their more characteristic aspects." Jackson 

 has made some luminous observations on the relation of these three 

 genera and on the influence of the character of attachment by 

 cementation in producing modifications in members of the Ostreidae. f 

 He believes that Gryphcea and Exogyra "are probably aberrant 

 members of the Ostreidae, not typical forms in the line of evolution 

 of the group." 



* Leymerie (la). See also Peron's remarks on these genera, Peron (1), 

 pp. 107-9. f Jackson (1), sections v. and vii. 



