464 BULLETIN 103, UNITED STATES NATIONAI. MUSEUM. 



appearing as truncated cones, sometimes compressed (at the side 

 of the corallum they are distorted), unequal in size; margins thin. 

 Fossa not deep, but variable. Columella well developed, projecting 

 at the bottom of the fossa; its component tissue is laminar and folded, 

 and it is rounded above. Septa straight, very slightly exsert, delicate 

 thi'oughoiit, not larger at any point decidedly; but the largest are 

 more delicate midway between the walls and the columella ; they are 

 arranged in six systems of four cycles. The prmiary and secondary 

 septa are equal; the tertiary a little smaller; those of the fourth 

 order are very small, and barely developed in some calices, but they 

 exist in all. The primary and secondary septa have a tooth near 

 the columella. Endotheca tolerably developed. Exotheca well 

 developed, forming large and small cells, both square, though often 

 divided by dissepiments. Reproduction by extracalicular gemma- 

 tion. There is no epitheca. 



"Dimensions. — Height of corallum several inches; diameter of 

 calices from a little less than 3 lines to 4 [6.25 to 8.3 mm.]; thickness 

 of septa one-sixtieth inch [0.4 mm.]. The dimensions of the ellip- 

 tical calices are — length, 3-^j lines [7.3 mm.] ; breadth, 2| Imes [5.2 mm.] ; 

 depth of fossa, two-thirds of a line [1.4 mm.]. Exothecal cells from 

 one-fourth to one-half line [0.5 to 1 mm.]. The lateral calices are 

 very irregular, and the younger corallites have thi-ee cycles of septa. 



" Fossillisation. — Calices, as a rule, not filled up. Sclerenchyma 

 light-brown in color, opaque, and siliceous, the central portions of 

 the corallum evidently consisting of dark homogeneous flint, the 

 sclerenchyma having been destroyed in the process of silicification. 



''From the Marl-formation of Antigua. Coll. Geol. Soc." 



Plate 130, figures 2, 2a, presents illustrations of Duncan's type (No. 

 12942, collection of the Geological Society of London). Duncan 

 was of the opinion that this species belonged to the genus Heliastraea 

 Milne Edwards and Haime, which is a synonym of Orlicella Dana, 

 It was my belief that the species was referable to Orbicella until I 

 obtained a number of remarkably good specimens in Antigua. A 

 selected series of these wiU be described in the following remarks : 



The corallum forms rounded or discoid masses, the two largest I 

 collected having the following dimensions : No. 1, horizontal diameter, 

 225 by 305 ram, ; height, 155 mm. No. 2, horizontal diameter, 322 by 

 400 mm.; height, 131 mm. Specimen No. 1 has a more arched 

 upper surface than No. 2 which is more discoid in shape. 



On the lower surface of the corallum there is very little epitheca — 

 only shreds in places. Costae are well-developed, subequal, inter- 

 rupted here and there; intercostal furrows perforate, many synap- 

 ticulae present, joining the outer ends of adjacent septa (see pi. 130, 

 figs. 1, la). 



