those organs, and that they probably represent the apodemata, or infoldings, of the hard external crust to which those organs, or the muscles by which they were moved, were attached.
The prominence of the hypostome in the Trilobita reminds one even more strongly of the genus Apus than of the Isopods ; and it is quite reasonable to expect, in the Trilobita, a more generalized type of structure than that which marks the modern representatives of the class.
The question will naturally be asked why so many specimens of Trilobites should be found, yet no traces of limbs. It seems reasonable to infer that a large number of these fossil remains are only exuvioe, the Crustacea frequently casting their shells. The detachment of the limbs is also a common occurrence in all the fossil Articulata, especially where, as in the Crustacea, the proximal joint is extremely constricted, and, in consequence, easily disarticulated.
Nor need, we assume that all the genera of this very extensive family had horny or chitinous limbs, seeing that in the modern Isopoda a great diversity exists in these organs.
With regard to the section of Calymene senaria exhibited, it seems difficult to accept the suggestion that these are the eggs of the same Trilobite, seeing that they fill the entire cavity of the test, as shown in the section.
The publication of Mr. Billings's discovery appears to me to be of the highest importance to palaeontologists ; and he is entitled to our best thanks ; for his observations are sure to excite further researches upon the Trilobita, and thus will be the means of greatly increasing our knowledge of this interesting group.
3. On the Structure and Affinities of Sigillaria, Calamites, and Calamodendron. By J. W. Dawson, LL.D., F.R.S., F.G.S., Principal and Vice-Chancellor of M'Gill University, Montreal.
(The publication of this paper is deferred.)
[Abstract.]
The object of this paper was to illustrate the structure and affinities of the genera above named, more especially with reference to the author's previous papers on the " Structures in Coal" and the " Conditions of Accumulation of Coal," and to furnish new facts and conclusions as to the affinities of these plants.
With reference to Sigillaria, a remarkably perfect specimen of the axis of a plant of this genus, from the Coal-field of Nova Scotia, was described as having a transversely laminated pith of the Sternbergia type, a cylinder of woody tissue, scalariform internally and reticulated or discigerous externally, the tissues much resembling those of Cycads. Medullary rays were apparent in this cylinder ; and it was traversed by obliquely radiating bundles of scalariform vessels or fibres proceeding to the leaves. Other specimens were adduced to show that the species having this kind of axis had a thick outer
