250 
Ivoninck has expressed an opinion that the two shells in q^uostion, referred by Morns to 
P. brachythmrus, arc different, and do not belong to the same species, one being possessed 
of a short septum in the dorsal valve and less marked muscular scars, the other, on the 
contrary, with a very long septum and strongly marked scars. In working out the 
species it became essential for anything like correct determination that some more 
satisfactory solution of this subject than mere opinion should be arrived at. 
The specimons used by Mr. Gr. B. Sowerby haying disappeared, as already 
explained,* * * § those contained in Strzelecki’s Collection in the British Museum, and 
described by the late Prof. Morris must be accepted as the types, and to their structure 
all future ajrpeals must be made in determining the identity of P. 5roc/iyf/iiBrMS, G-. Sow., 
Morris, blow, as stated above, Prof. De Koninck regards the silieified castf with the 
long septum, described by Morris as P. hrachyilimrus, to be specifically distinct from the 
true P. hraaliyth(Brus,X a name which ho considers should be retained for a form with a 
short septum, amongst other characters. The silieified cast figured by Morris in 
Strzeleeki’s Work (PI. xiv., ff. 4a and h) has, for one of its most distinctive characters, a 
long septum in the dorsal valve, as previously pointed out ; but the example of P. 
'brachyilimrus represented by the sandstone cast accorded much bettor with Sowerby’s 
description of his species, than did the siliceous example. The specimen in question, J as 
then exposed, was that of a ventral valve backed up with matrix so that on the removal 
of the latter the cast of the dorsal valve would be visible and the septum exposed, long 
or short as the case might be. It was found to possess, so far as we are able to judge, 
a short septum, so probably confirming in a remarkable manner Prof. De Koninck’s 
surmise. The matter then stands thus: The figure given by Morris in Strzelecki’s Work 
on “ New South Wales,” PI. xiv., f. 4c, is the true P. hrachythcerus, characterised by the 
presence of a short septum and a little developed muscular system. On the other hand 
(PI. xiv., ff. 4a and b, of the same Work), the siliceous cast, with a very long septum and 
great muscular development, is a distinct and separate species. McCoy’s typo specimens 
of P. hrachyth(Brus, in the Woodwardian Museum, Cambridge, do not call for any 
particular notice beyond the fact that they show the species to be in outward appearance 
not Tinlike the variety pugilis, Phill., of the European species, P. semireticulaius, 
Martin, and a peculiar strcalcy appearance is given to the exterior of the shell by 
the decurrent bases of the spines. With regard to McCoy’s P. undulatus,^ Prof. 
De Koninck II refers it to P. bmcJi-ythterus ; but a careful examination of the 
type in the Woodwardian Museum has not convinced me of their identity. I have 
not seen such peculiar undulating strice upon any specimen of P. hrachythmrm I have 
examined. 
When well preserved, and not crushed or broken, P. brachytlimrus is a subquadrate 
shell, high and gibbous in the visceral region, rapidly and deeply sloping off at the 
sides to the flat, triangular ears. Such a condition is represented by Morris’s original 
figure,^ one of De Koninck’s of his so-called P. aubqiiadratus, Morris,** and his earlier 
figure of P. hracliythwrusXX and his last figure of the same species. The ears of this 
species appear to have been particularly fragile, and more often than not have com- 
* Proo. JK. Phys. Soo. Edinb., 18S0, v., p. 28y. 
t Strzeleclii, loe. c^^., PI. 14, f. 4a and 6. 
J Strzelecki, toe. cit., PI. 14, t. 4c. 
§ Ann. and Mag. Nat. Hist., 1847, xx., p. 235. 
PoS3. Pal. Nouv.-Galles du Sud, 1877, Pt. 3, p. 198. 
IT Strzelecki, !oc. cit, t. 14, {. 4c. 
** Mon. Gen. Productu.s, t. 14, f. Ic. 
if Idid., t. IG, f. la and 6. 
fJPoss. Pal. Nouv.-Galles du Sud, 1877, Pt. 3, t. 10, f. 4a and b. 
