4,54s 
the before-mentioned shells to one species, and rash statements of the kind quoted do 
not tend to assist in unravelling the question. I have retained MaccoyeUa BarMyi, 
M. rejlecta, M. uinbonalis, M. corbiensis, and M. ? substriata. 
It is even possible that an additional species may exist. PI. 24, fig. 1, represents 
aright valve* devoid of the posterior wing, without cost®, but possessing strong con- 
centric lamiuffi and the general peculiar shape and shell-structure of a Maccosjella. It 
seems distinct from either of the other species placed in this genus. 
The hinge structure of the various species of Maccoyella, although appearing to 
differ widely at first, is found on examination to follow a well-marked plan, and afford 
a really excellent example of palfeontological morphology, from the semi-pectinoid 
outline and restricted and convoluted hinge of M. Bas'khji and ilf. refif-cia to the 
gryjihma-form sha])e, and small area of M. corbiensis. Between these two extremes 
stands M. iimhomlis, acting as a connecting link. In M. Barkhji and ilf. rejlecta the 
umbonal fold is thick and projecting, but short, the byssal fold on the contrary being 
much longer and projecting inwards, with a very deej) and narrow byssal sinus anterior 
to it. The hinge is thus much broken up, and does not present the same regularity of 
outline as seen in M. umbonalis and M. corbiensis. In the right valve of the first of 
these species, being that with which I am best acquainted, the dorsal margin is long 
and obliquely bevelled, the area comparatively flat with a decided submodian cartilage 
depression, and the folds, umbonal and byssal, prominent, but not nearly to the same 
extent as in the two former speeies. The umbonal is nearly vertical, with the larger 
portion of the concave cartilage depression posterior to it. The byssal fold is less 
prominent still, wherein this species departs greatly from the typical structure of 
II. BarMyi and Jlf. rejlecta., and the byssal sinus is tolerably long but not very deep. 
In ilf. corbiensis the hinge structure least roseinbles that of the genua of any of its 
species. The dorsal margin of the left valve is much arched, widening out below the 
umbo into a rather triangular, concave, cartilage area, divided excentrically by the 
umbonal fold, the byssal fold being obsolete. 
The hinge structure of the smaller species referred to Maccoyella is unknown to me. 
The structure of the test in Maccoyella is non-prismatic, and apparently laminar- 
fibrous. Thin sections show no trace of the prismatic-cellular structure of the 
Aviculid® ; but as I cannot be certain that the actual outer layer has been examined, 
a decision on. this point must be suspended. It is probable that the interior of the 
valves was nacreous. The surface of the best-preserved examples examined by me 
shows a hackly-laminar appearance irrespective of the outside ornament. 
The distribution of Maccoyella is very extensive. The original localities recorded 
by Moore are Wollnmbilla and Mount Corby (Bolling Downs). Species have since been 
obtained by the Hon. A. C. G-regory at the alsh River (Rolling Downs) ; by Messrs- 
Jack and Rands at Maryborough (Desert Sandstone) ; probably also by the latter at 
the Isis River (Desert Sandstone) ; by Messrs. Jack and Sam well at the Croydon 
Gold Field (Desert Sandstone) ; and probably by Mr. Jack at Minmi, near Roma 
(Rolling Downs). Numerous examples have been collected by Mr. H. Y. L. Brown 
in the Lake Eyre District, South Australia ; whilst Prof. R. Tate has recorded the 
presence of the genus in the Peak District of the same Province. Finally, as 
intermediate between the Queensland and South Australian localities, the late Rev. 
J. E. T. Woods has figured examples from the Gray Ranges, on the borders of this and 
the former Colony. 
Type — Avicula Barklyi, Moore. 
It was collected by my Colleague at Richmond Downs, Flinders River, from the Rolling Downs Formation. 
