DESCRIPTION OF CHALK CORALS. 271 
proofs of disturbed growth or mutual interference. The uniserial mode of growth 
with developments from the upper extremity of the fixed portion of the tube 
demand however careful consideration ; and every deviation should be duly in- 
vestigated, with a view to ascertain whether an extension of generic characters 
should be proposed. 
Diastopora Sowerbii. (Tab. XVIII. A. figs. 2, 2 a.) 
Disks numerous, united by mutual extension, and forming successive layers ; 
visceral tubes long, cylindrical, exposed portion very variable, sometimes con- 
siderable, sometimes small, concealed in aged conditions ; apertures round or 
oval, closed in advanced stages, slightly inclined upwards, edges projecting. 
In M. Milne-Edwards’s valuable memoir on the genus Diastopora' the species 
are divided into three groups, the first consisting of those which develope a single 
layer ; the second, of species which produce overlying, successive layers ; and 
the third, of such as are formed of two layers placed back to back. This arrange- 
ment, simple in itself, requires however a good illustrative suite of specimens 
before, in many cases, a species can be determined, or a reference made to a group 
when a single example is the subject of examination. Thus Diast. diluviana, as 
described by M. Edwards, consists of numerous extended layers’, which envelope 
each other like the coats of an onion, and every successive layer conforms to those 
inferior to it, the origin of each resembling a little circular crust. It would 
consequently be impossible to decide to which of the two first divisions a primary 
disk belonged, or to assign it positively to a known species. The chalk fossil 
under consideration presented many signs of more than one layer; but no evi- 
dence of its earliest condition. From Diast. diluviana, as well as M. Michelin’s 
D. microstoma’, it differs in the many centres of development or limited exten- 
sion of each. Respecting the nature of the fundamental layer only a conjecture 
can be offered, but it possibly consisted of a single disk ; and it is believed, that 
the actual surface of the specimen (fig. 2) arose, in the first place, from the de- 
velopment of two or more centres on that disk, the lateral extension of which 
beyond the original boundary producing a greater area; and secondly from one 
‘ Annales des Sciences Naturelles, 2de série, Zool. tome ix., or Recherches sur les Polypes, Mém. 
sur les Crisies, &c. p. 31. 
* Op. cit. pl. 14. fig. 4, and pl. 15. fig. 3 to 3c. Berenicea diluviana, Lamouroux, Exp. Méthod. 
p. 81. Consult his figs. 3, 4, tab. 80, for an example of a simple disk. 
* Iconographie Zoophytologique, pl. 57. f. 1. 
2Nn 2 
