251 
pletely disappeared. When this is the case we find the shell assumes a subtrapczoidal 
outline, and presents a very difEeront appearance to that previously described. In 
this state it is represented by another of De Koniiick’s figures of P. suhquadratus* 
and my own from the Bowen Eiver Coal Pield.f At times a highly geniculate 
form 18 assumed by P . hraclythcerus, with various intermediate conditions between this 
Md the former outlines. When much reflected in this way it becomes the P. fragilis, 
Dana, J and one of the intermediate forms is shown in another figure of my own from 
North Queensland. § ^ In the Mining and Geological Museum, Sydney, is a very 
geniculate individual from the Bowen Kiver Coal Field, in which the depth through 
the shell is two inches. These are the forms usually assumed by P. Iraclmjtliwrm. 
The elongated spine bases are clearly contained within the substance of the shell, and 
when decortication takes place, the spine bases appear to be removed with the outer- 
most shelly layers, leaving long open channels. For quite a third of the height from 
he front margin, these spine bases are absent, and this portion bears scattered out- 
standing spines, the intermediate portions being glossy and smooth. 
Now, with regard to the reference of Froducius suhquadratus (Morris), De 
oninck, and P. fragilis, Dana, to this sjiecies. P. suhquadratus, Morris, || was very 
briefly described— too briefly, in fact—and in the absence of a figure would have been 
unrecognizable, had it not been for the fact that Morris says— “Mesial furrow broad 
and distinct.” Herein it differs entirely from P. hrachyiharus. The only figures extant 
referred to P. suhquadratus are those published years ago by De Kouinck,^ one of which 
is an ordinary form of the P. hrachgthwrus, a second is that form of this species with 
the ears broken off, as I have above described : and the identity of the third is doubtful. 
Fhere can be no doubt about the second figure, the short (broken), erect, sharp hinge, 
trapezoidal outline, short septum, and no trace of a sinus, arc characters not to be mis- 
taken. It is but just to Prof. De Koniuck to say that he even doubted the accuracy of 
his own determination, for he says— “ Je ne suis pas entierement eonvaincu de la rSalite 
de Cette espece, et je le n’adopte provisoirement qu’en faisant mes reserves pour I’avenir.” 
Froducius fragilis was the name given by Prof. J. D. Dana ** to the highly 
geniculate condition of P. hracliythwrus, an opinion I have previously expressed ft and 
s ill adhere to. Prof. De Koninck held a contrary view, and has furnished an elaborate 
description of wdiat he believed to be F. fragilis, Dana, in his recent Work.JJ 
So little reliance can be placed on some of the late Prof. De Koninck’s descrip- 
lons, this amongst the number, that it is extremely difficult to grasp the requisite 
Characters for the detection of the species then running in the mind of the Author. He 
states that the dorsal valve of F. fragilis is “legerement concave” ; but some few lines 
urther on, on the next page, when comparing the species with P. hrachytlixrus, he 
luliifies this statement by saying, “la convexitb de sa valve dorsale.” It is quite 
possible, as he observes further on, that the shell he is describing as F. fragilis may be 
eiitical with the unnamed siliceous casts in Strzelecki’s work, previously referred to 
\ -14, f. 4f?-fi). Of the true F. fragilis, Dana, however, I am in no doubt. I fail to 
loiv it can bo anything more than a variety of the species now under description. 
*,Mon. Gen. Productus, t. 11, f. Id. 
tProc. E. Phys. Soc. Edinb., 1880, v., t. 8, f. 10. 
t Geology Wilkes’ U. S. E.xplor. Exped., 1849, Vol. x., Atlas, t. 2, f. 17. 
%Loc. oit., t. 9, f. 17 and IS. 
II Strzelecki’s Phys. Descrip. N. S. Wales, &c., 1845, p. 284. 
IT Mon. Gen. Productus, p. 203, t. 14, f. la-d. 
** Geology WUkos’ U. S. Explor. E.xped., 1849, Vol. x., ] 5 . 686, Atlas, t. 2, f. 7. 
tt Proc. E. Phys. Soc. Edinb., 1880, v., p. 287. 
ff Foss. Pal. Nouv.-Galles du Sud, 1877, Pt. 3, p. 201. 
