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The structure and zoological position of Receptaculites have been more 

 or less elaborately investigated by Goldfuss, Eichwald, Roemer, Salter, 

 Hall, and other eminent observers, and yet, owing to the imperfection of 

 the materials, a great deal remains to be done before the various ques- 

 tions involved in the relations of this curious genus can be regarded as 

 positively settled. Since the publication of Salter's paper in the first 

 Decade of our Geological Survey, numerous specimens of several distinct 

 species have been collected in the Silurian rocks of Canada, and I am, 

 by the study of these, now enabled to furnish a few additional details. 

 The principal new points are, the perforated structure of the internal inte- 

 gument, the existence (in most, if not in all, of the species) of a great 

 central cavity and an orifice in the upper side. The flat watch-shaped 

 specimens which are usually figured as constituting the whole of the body, 

 are probably only the basal portion of the body-wall of the discoid species. 



The genus may be described as consisting of organisms, which, when 

 full grown and perfect, are of a discoid, cylindrical, ovate, or globular 

 shape, hollow within, and usually, if not always, with an aperture in the 

 upper side. In or near the centre of the lower side there is generally to 

 be seen a small rounded protuberance, indicating, most probably, the posi- 

 tion of the primitive cell or nucleus from which the animal commenced 

 its growth. In some species the lower side is more or less concave, and 

 often the nucleus is not at all elevated above the surface adjacent thereto. 

 Its place, however, in the absence of any other guide, may generally be 

 found by observing the point towards which the spiral lines or rows of 

 plates on the outer surface converge. The body-wall is of a somewhat 

 complex structure. It consists of three parts, an external and an 

 internal integument, and, between these, the peculiar tubular or spicular 

 skeleton presently to be described. The external integument may be 

 called ' the ectorhin,' and the internal ' the endorhin.' 



The ectorhin is usually composed of numerous small rhomboidal plates 

 closely fitting together, and arranged in curved rows which radiate in all 

 directions from the nucleus outwards to the peripheral margin of the base, 

 and thence, ascending upwards, converge to the edge of the aperture in 

 the upper side. Two or three of those rows of plates (the precise num- 

 ber is not yet determined) originate in the nucleus, and, as they diverge 

 from each other, new rows are introduced between them. The rows 

 diminish, in number, again on the upper side according as they converge 

 towards the apex of the fossil. The plates at and immediately around 

 the nucleus, and also towards the centre of the upper side, are somewhat 

 smaller than they are at the widest part or middle region of the body. It 

 seems probable that, in some of the species, this integument was of a flex- 

 ible, coriaceous consistence. In others the plates were solid. In It. 



