1865.] BILLINGS— SILURIAN AND DEVONIAN FOSSILS. 185» 



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, in- 

 dicating, most probably, the position of the primitive cell or nu- 

 cleus from which the animal commenced its growth. In some spe- 

 cies 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 some- 

 what complex structure. It consists of three parts, — an external 

 and an internal integument, and, between these, the peculiar tubu- 

 lar or spicular skeleton presently to be described. The exter- 

 nal 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 number is not yet determined) originate in 

 the nucleus, and, as they diverge from each other, new rows are in- 

 troduced between them. The number of rows diminish 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 flexible, coriaceous consistence. In others the plates were 

 solid. In R. occidentalis (Salter), when silicified specimens are 

 treated with acid the plates are easily separable, and, therefore, 

 although in close contact, they were not anchylosed together. 



The endorhin is also composed of small rhomboidal plates ar- 

 ranged in curving rows ; but it differs from the ectorhin in being 

 perforated by numerous small circular orifices, one of which is 

 situated at each point where the angles of four plates meet. From 

 the centre of each of the plates of this integument there radiate 

 four small canals, one proceeding straight to the middle of each of 

 the sides of the plate, where it communicates with a similar canal. 



