113 



times almost obsolete. The species differs from P. polymor- 

 phoides in its fewer ribs, never bifurcating or presenting 

 intercostal ribs towards the front margin, and by the regular 

 convexity of its valves. 



Pecten polymorphoides, Zlttd. Plate viii., fig. 2. 



Reference. — Pal. von Neu-Seeland, p. 51, t. xi., fig. 3, 1864. 



iSynonyms. — Pecten incertus (Tenison- Woods), Proc. Phil. Soc, 

 Adelaide, 1865, tab. i., fig. 1. P. coarctatus (^Sturt), Two 

 Exped., vol. XL, t. iii, f. 13 ; id. Ten.-Woods, in Geol. Obs., 

 p. 74, fig. 



Shell triangularly orbicular, inequivalve, very inequilateral, 

 projecting anteriorly ; convex, irflexed in front ; umbonal part 

 irregularly rudely tumulous, rayed with five to seven rather 

 broad ribs ; ribs and sulci longitudinally ridged. The ridges, 

 which are covered with imbricating scales, increase in number 

 by bifurcation towards the front, and are there grouped more 

 or less in pairs ; interstices between ridges concave, regularly 

 transversely striated ; front edge of valve crenulate-dentate. 

 Ears of left valve very unequal ; the anterior one is of mode- 

 rate size, abruptly truncated, with a few scaly rays ; the pos- 

 terior one is elongated, with four stout, coarsely scaly rays ; 

 byssal sinus narrow and deep ; adductor scar narrow, placed 

 high up on the anterior area. 



Dimensions. — Length, 4i ; height, 49 ; thickness through 

 both valves, 16 millimetres. 



Localities. — Middle Murravian, near Morgan, Blanchetown, 

 and Mannum ; Muddy Creek, Hamilton {B. T.). Also m the 

 Oamaru series, New Zealand. 



P. polymorphoides exhibits some variability in respect to the 

 development of the radial ridges and the corrugations in the 

 umbonal region, and to the extent of the inflexion of the front. 

 Young specimens are nearly equilateral ; Zittel's figure repre- 

 sents the species at an early stage of growth, and in which the 

 radial ridges and the ventral inflexion are pronounced ; his- 

 description does not well apply to the adult shell in respect to- 

 the subsmooth or obsoletely striated plicje, and the auricles- 

 as figured by him are not exact. Tenison- Woods' figure of his 

 P. incertus represents an averagcd-sized specimen, but without 

 corrugations. A specimen of P. polymorphoides of about 25- 

 millimetres, sent to me by Professor Hutton, removes any 

 doubt as to the specific identity ol the two shells. P. Burnetii 

 (Zittel) is a closely-related species. 



Among living congeners, P. polymorphoides is related to- 

 P. convexus, Quoy and Graimard (P. roseopunctatus, Reeve, and 

 P. vellicatus, Hutton), but its more numerous scaly and slender 

 rido:es and the umbonal corrugations distincjnish it ; and 



