SUB- GENUS HETEROTRYPA. 113 



than in the large ones. The " monticules " do not appear, as a 

 rule, to differ in structure from the general mass of the coral- 

 lum, but they seem sometimes to comprise a larger proportion 

 of small tubes than is usually the case in the intervening parts 

 of the skeleton. In the axial region of the corallum, as shown 

 both in longitudinal and transverse sections, the corallites are 

 thin-walled and polygonal, but they possess an abundance of 

 tabulce. In the outer portion of their course, the corallites 

 become thickened, their walls assuming a fibrous and laminated 

 structure (fig. 18, e). 



The internal structure of M. [Heterotrypa) riigosa, E. and H., 

 as I have elsewhere pointed out (Ann. Nat. Hist, ser. 4, vol. 

 xviii. p. 88), appears to be essentially the same as that oi AI. 

 raniosa, D'Orb. ; and a careful examination of thin sections of 

 the form which I take to be the M. Dalii of Edwards and 

 Haime {^Chcetetes approximatus, Nich.) has satisfied me that 

 the same is the case in this instance also. I shall therefore 

 insert here a brief description of the two types just alluded to, 

 regarding them merely as well-marked varieties of M. rainosa, 

 D'Orb. 



Var. I. — M. ramosa, D'Orb., var. rugosa, E. and H. 



(Fig. ig, A and v.; and PI. II. fig. 3.) 



A[ojiticHlipora rugosa, Dybowski, Die Chastetiden, p. 92, PI. III. fig. i, 1877. 



Obs. — This form was originally defined as a distinct species 

 by Milne-Edwards and Haime (Pol. Foss., p. 268, PI. XX. 

 figs. 6, 6ci), and is at first sight readily distinguished from 

 typical examples of M. rainosa, D'Orb., by its external char- 

 acters. In place, namely, of the conical monticules of the 

 latter, the surface now exhibits numerous well-defined eleva- 

 tions, which are transversely elongated, so as to constitute so 

 many discontinuous transverse ridges (PI. II. fig. 3). These 

 ridges vary in length ; but they do not extend round the stems, 

 and are usually sharp-edged, and are placed about half a line 



II 



