120 Annals of the South African Museum. 



1879. Trigonia conocardiiformis J. Lycett, Mon. Brit. Foss. Trigoniae, 

 pp. 210, 211, and woodcuts; p. 230. (Palaeontographical 

 Society.) 



1903. Trigonia conocardiiformis C. Burckhardt, Beitrage zur Kennt- 

 niss der Jura- und Kreideformation der Cordillere. Palse- 

 ontographica, Band 1., Taf. xiii., figs. 3-5. 



Supplementary Descriptive Note. Krauss gave an admirable 

 detailed description of this very characteristic shell, but I am able to 

 supplement this by some additional observations, based on the 

 examination of material more favourably preserved than that which 

 appears to have been at his disposal. There are one or two points, 

 also, in which the figures accompanying Krauss's description are 

 rather misleading. 



With regard to the ribbing of the flank, it must be noted that this 

 does not always strictly conform with the simple plan illustrated in 

 the figures given by Krauss and Lycett. It may be remarked that 

 if the figures of the German author are imperfect, that given by 

 Lycett to illustrate the exterior of a left valve is still less character- 

 istic, although it was intended to supply the deficiencies of the 

 earlier illustrations. 



Several specimens now examined are in such condition as to show 

 the form of the sculpture in early growth-stages, and two individuals, 

 in particular, have the ornaments of the nepionic and neanic stages 

 well preserved. One is an imperfect left valve which measures 

 9 mm. in length, and would be 7 mm. in height if uninjured at the 

 pallial border ; it occurred in a hand-specimen, containing adult 

 T. conocardiiformis, T. vau, and other typical forms, from the 

 Sunday's Eiver. The other specimen is an immature individual 

 having both valves in place, imperfect posteriorly, but beautifully 

 preserved in the umbonal region. This was collected by Mr. Eogers 

 from a cliff on the right bank of Sunday's Eiver on Commando 

 Kraal (104h). Other specimens, also, show the ribbing of the neanic 

 stage, though less perfectly. It is seen that until the valve attained 

 a height of about 6 mm., measured from the umbonal apex, the 

 flank was ornamented by simple concentric ribs, running parallel to 

 the lower margin. Close to the apex the crests of the ribs are 

 situated at a distance of about 0'5 mm. apart ; at about 5 mm. 

 below the apex the distance between the ribs has increased to 1 mm. 

 These concentric ribs extend to the frontal margin. Traced back- 

 wards, they pass over the flank to a definite carinal ridge, and 

 becoming more delicate in character, turn sharply forwards and 

 pass across the relatively narrow area. There is no inner carina 



