122 Annals of the South African Museum. 



part by being sunk in concave form. Towards the posterior end it 

 is scarcely demarcated. 



Dimensions. Trigonia conocardiifonnis may attain very large 

 dimensions. A specimen presented by Atherstone to the Geological 

 Society of London has a maximum length of 164 mm. This 

 specimen, which is not quite complete at the lower margin under 

 the umbonal region, must originally have had a height (measured 

 at this part) of 95 mm. The ligament groove is 45 mm. long. A 

 specimen figured by Krauss is 84 mm. in length and 42 mm. in 

 height (at the umbonal region), while the depth of a single valve, 

 measured from the figure, is 24 mm. In several individuals 

 examined by me, the height is somewhat greater than this, in 

 relation to the length. 



Occurrence. This form is found in the Marine Beds at various 

 localities on the Sunday's and Zw-artkop's Eivers. It was obtained 

 by Messrs. Rogers and Schwarz from the railway cutting between 

 milestones 24^-24-J on the line from Uitenhage to Graaff-Reinet 

 (297, 298) ; also at the white krantz on Wolve Kraal on the north 

 bank of Sunday's River. The same authors have mentioned the 

 occurrence of this shell in a conglomeratic bed at Plettenberg's 

 Bay.* Specimens in the collection of the South African Museum 

 are from the Sunday's River. In 1905, Mr. Rogers obtained 

 examples in the cliff on Commando Kraal, right bank of Sunday's 

 River (104h) ; in the highest beds on Zoet Geneugd, right bank of 

 Sunday's River (67h) ; on a bare slope W. 30 S. from the middle 

 of Barkly Bridge, on the farm Olifant's Kop, Sunday's River (21h) ; 

 and from the Nek S. 33 E. from Comley's house, right bank of 

 Sunday's River (86h). 



Remarks. A very striking feature exhibited by this Trigonia is 

 the sudden transition of sculptural characters at the close of the 

 neanic stage. The abrupt manner in which the crowded concentric 

 ribs give place to coarse, widely spaced, inclined ribs, recalls the 

 analogous transition in Trigonia vau and T. stowi ; in these, the 

 youthful stage has similar though more delicate concentric orna- 

 ments, and these are replaced almost as abruptly by angularly bent, 

 coarse ribs. The youthful characters of T. conocardiifonnis seem 

 perhaps to suggest an ancestry similar to that from which T. vau 

 and its ally were derived. In the young T. conocardiifonnis, 

 however, the carinal ridge is more clearly defined, and in passing 

 across this the ribs are sharply bent to form an angle. The early 

 characters do not seem to indicate alliance with either the Scabrae 



* Schwarz (1), pp. 53, 61 ; Kogers and Schwarz (1), p. 5 ; Rogers (1), p. 295. 



