SUB GENUS HETEROTRYPA. 105 



inches in height, consisting of two layers of corallites, which 

 diverge from an imaginary central plane to open on both sides 

 of the frond. (Occasionally assuming a massive form ?) Sur- 

 face covered with rounded, conical, or elongated elevations or 

 " monticules," which are sometimes but slightly raised above 

 the general surface, whilst in other examples they are conspic- 

 uously elevated. These monticules are composed of corallites 

 which sometimes appear to be slightly larger than the average, 

 and which at other times are decidedly smaller than the ordi- 

 nary tubes ; while in some specimens the sides of the monticules 

 are covered by full-sized tubes, and the summit is occupied by 

 smaller corallites. The distance between the monticules is 

 mostly from half a line to a line. Ordinary corallites of two 

 kinds — large and small. Large corallites, polygonal or sub- 

 polygonal, from i-iooth to i-8oth inch in diameter, their walls 

 amalgamated with one another, and moderately thickened as 

 they approach the surface. Small corallites moderately but 

 not excessively numerous, intercalated in the intervals between 

 the large tubes, variable in size and shape, but always angular 

 or sub-angular. A variable, but often considerable, number 

 of minute thick-walled circular tubuli (" spiniform corallites ") 

 developed between the normal corallites. Large corallites 

 with comparatively few and remote tabulce ; small corallites 

 with numerous close-set tabulae ; all the tabulae complete and 

 approximately horizontal. 



Obs. — This well-marked species — the identification of which 

 with M. maiiwmlata, D'Orb., will be subsequently discussed 

 — resembles M. frondosa, D'Orb., M. molesta, Nich., and M. 

 Dawsoni, Nich., in being normally frondescent (PI. VL fig. i), 

 and in consisting of two strata of corallites which diverge in 

 opposite directions, and nearly at right angles, from a median 

 plane, which may be considered as dividing the corallum into 

 two leaf-like halves. This median plane, however, is an Ima- 

 ginary one, not marked by any mesial calcareous lamina ; so 

 that, In this respect, the structure of the corallum differs from 

 what we shall see to exist In M. frondosa. The calices are 



