253 
circumscribed by tbe strong prominent encircling ridges, higher than wid( 3 , and vertically 
dendritic; reniform impressions oval, large, and faintly marked, the limbs quite 
horizontal. Surface of both valves with alternate rows of tear-like tubercles, arranged 
roughly in quincunx, giving support to short free spines, the whole interrupted by a few 
concentric undulations. 
Ols. The above characters are taken both from specimens with the outer 
shell more or less preserved, and excellent internal casts. It occurs in some profusion 
near Mount Britton Township, in the latter condition (PI. 38, figs. 7-10) exhibiting 
many of the characters in great beauty. 
This is probably the largest of the Australian Producti, even exceeding P. com 
in size, which it resembles somewhat in outline. It is remarkable for its massive and 
geniculate form, wide open sinus, and straight wall-like sides, characters to some extent 
commented on by Morris. 
The umbo of the ventral valve is fairly large for the size of the shell, doubtless 
to provide room for the large proximal end of the cardinal process, and although 
incurved, is not produced to overhang the hinge-line. Without doubt there is an area 
in both valves, which is always short and triangular in the ventral, but seems to be 
longer in the dorsal. That of the former is excellently shown in its most exaggerated 
form in a specimen from Yattou, in which nearly the whole of the shelly matter is pre- 
served. The area of both valves, when united, is seen in several specimens from Mount 
Britton, but more pai’ticularly on the impression of a large valve, three by three and 
a-half inches in size, from Lake’s Creek. Here the dorsal margin is thickened, forming 
an area nearly one-eighth of an inch wide, and delicately transverselv striate. This, 
and the triangular pseudo-deltidial aperture, are characters which do sometimes occur 
m Productus proper, as, for instance, in the specimen of P. setnireticulatus figured * by 
the late Dr. T. Davidson, from the English Carboniferous Limestone. He remarked on 
this “ Possesses sometimes, although rarely, a well-defined area and fissure covered by a 
pseudo-deltidium.” The presence of the blunt jirotuberanccs, one on each side of the 
fissure, have already been referred to in the Yatton shell, and it might be argued that 
from the presence of those we are dealing with a Slrophalosia ; but the reniform impres- 
sions are clearly those of Froduatus, so that any reference to the former must be 
^bandoned. Could we be certain that these callosities represent hinge teeth received 
tnto sockets on the opposite, or dorsal valve, the genus Productella, Hall,t would put in 
^ much stronger claim. In this genus there are a “ narrow area on each valve, a foramen 
^ callosity on the ventral area, small teeth, and more or less distinct teeth sockets.” 
nt I am by no means sure that these thickenings are any more than so, and do not 
Actually represent hinge teeth in the sense of those seen in Sirophalosia&nA Productella. 
The adductor muscular scars of the ventral valve are placed low down, and are 
endritic (PI. 40, fig. 5). In Productus, as a rule, these scars are elongately reniform, 
f®"^ species, such as P. humerosus\ and P. lineatus, Waagen, they resemble 
e examples now before us. Taking, therefore, the general structure of P. suhquadratus, 
1 will be found to depart considerably from the restricted description of Productus, and 
^arks a transition towards those ponderous and abnormal shells called Productus 
‘^omoides and P. llangollensis, by Mr. Davidson. § In Dr. Waagen’s later classification 
® the ProductidcB, these are made the types of a new genus Paviesiella,\^ and rightly so. 
* Mon. Brit. Garb. Brach., Pt. 4, t. 43, f. 5. 
+ Pal. N. York, 1867, iv., p. 153. 
X Mon. Brit. C.arb. Brack., 1861, Pt. 4, p. 147, t. 36, f. 1 and 16. 
• § Loc. cit., 1862, Pt. 5, t. 56, f. 8 and 9. 
II Pal. Indica (Salt Range Fossils), 1884, Ser. xiii., Vol. i., No. 4, fas. 4, p. 018. 
