199 ^ 



NOTES REGAEDING CERTAIN FOSSIL SHELLS 

 OCCURRING AT TABLE CAPE, SUPPOSED TO 

 BE IDENTICAL WITH LIVING SPECIES. 



By R. M, Johnston, F.L.S., etc. 

 [Read April 8, 1884.] 



In a paper prepared by me for this Society in the year 1879, 

 I drew attention to the fact that the per-centage of living to 

 extinct species was becoming smaller the more the beds were 

 investigated, and I then ventured to state that on the basis of 

 the per-centage method the fauna indicated the age of the 

 Table Cape beds to be rather eocene than miocene. 

 Recently Prof. Tate has taken up the investigation of the 

 supposed living species, and he informs me that a considerable 

 proportion of them have been compared with original types 

 and have been found to be new species. I need not anticipate ' 

 him in this matter, however, as he intends to communicate a 

 paper on the subject to this Society. For the present I bring 

 under notice that I have recently compared the living Pectun- 

 cuius laticostcdus, of New Zealand, with the fossil shell sup- 

 posed to be identical with it occurring abundantly in the 

 Table Cape beds, and, as anticipated by me five years ago, I find 

 that the latter form is quite distinct. Splendid specimens of 

 the living species were kindly forwarded to me from New 

 Zealand by Prof. Hutton. I submit a full description of the 

 Table Cape species as follows : — 



Pectunculus M'Coyi, Johnston. — Peciunculus laticostatus 

 of Tenison Woods and Prof M'Coy. 



Shell orbicular, convex, slightly broader than long, some- 

 what flattened towards beak, subtrigonal when young. Surface 

 with 29 to 31 regular radiating convex ribs separated by 

 somewhat narrower intersj)aces ; ribs broadening and becoming 

 less convex towards the margin ; marginal extremities of ribs 

 rarely obsolete in old specimens ; whole surface finely shaply, 

 striated concentrically ; hinge teeth, generally 10 on each side 

 the three nearest beak smaller and frequently obsolete on one 

 or both sides in old specimens ; inside of margin sharply denti- 

 culated; ligamental area depressedly triangular with close V 

 shaped striae, — in young specimens, 6 — 7 — in specimens of 

 about 2f inches long, 8 — 9, the anterior side of beak having 

 generally one stria more than the posterior side. 



Mature specimens 2 J to 3 inches long. This shell, hitherto, 

 has erroneously been referred to P. laticostatus, Quoy. 



It is very abundant in the Table Cape beds and is identical 

 in every respect with specimens of the same age received from 

 Cape Schanck. A specimen sent by Professor Tate from South 

 Australia is also identical in every respect. The species des- 

 cribed approaches more closely in most respects to the existing 



