1909. ] IN BRITISH SPECIES OF CORALS. 291 
Tracey Jackson * has shown how generaliy this takes place among 
plants and is exhibited by the leaves. In many plants these show 
progressive development when traced from the proximal to the 
distal end of the branch, and the stage at the proximal end is an 
earlier one than that of the leaf subtending that branch. A similar 
case has been suggested among Polyzoa, concerning the change 
in general shape of the zocecium‘t. Reduced to its simplest 
expression, branching in corals is seen to become simple fission 
in the vertical plane, exhibited by such a form as Zhecosmilia 
[Chorisastrea| rugosa (Tomes) non Laubet, which is only a 
Montlivaltia which has taken to division by fission; the Montli- 
valtia-stage of this coral is very near J/. painswicki Duncan §, 
which comes from the same horizon and district—the Aalenian 
of the Cotteswolds. Another case is the specimen figured by 
Duncan as Thecosmilia obtusa|| (d’Orbigny), which is only a 
Montlivaltia fairfordensis 4 Tomes, which has begun to divide by 
fission: both come from the Bathonian clay of Fairford, Gloucester- 
shire, and these forms as well as fully divided ones are common 
enough among the material from there. The examples quoted 
are exhibited in the British Museum, where all the specimens 
mentioned in this paper, unless otherwise stated, are to be seen. 
In some Ceelenterates fission normally occurs in the horizontal 
plane, resulting in the well-known phenomenon of strobilization. 
It is obvious that if horizontal fission were to occur in a coral, the 
upper product would obliterate the lower, since, having no means 
of locomotion, it would continue to grow in place and would 
secrete its skeleton on the top of the old **. That this new skeleton 
would be in strict continuity with the old is rendered likely by 
the soft nature of the secretory surface, which by its own weight 
would be moulded on the underlying old skeleton, and, when 
secreted, the new skeleton would reproduce the shape of, and so 
fit, the old. An interesting case showing this is afforded by a 
specimen exhibited in the British Museum (No. R.9148) of the 
Montlivaltia-stage (M. fairfordensis Tomes) of Thecosmilia obtusa 
(d’Orbigny), growing on an oyster with a ribbed shell. The ribs 
of the oyster-shell are continuous with the coste of the coral 
which has settled on it. 
* R.T. Jackson, 1899, “ Localised Stages in Development,’ Mem. Boston Soc. 
Nat. Hist. vol. v. no. 4, pp. 92, 131. 
+ W. D. Lang, 1905, Geol. Mag. pp. 259, 260. 
{ R. F. Tomes, 1882, Quart. Journ. Geol. Soc. p. 428; B.M. no. R. 10847, 
§ P. M. Duncan, 1872, Mon. Pal. Soc., Part ui. p. 17, pl. 1. fig. 12; B.M. no. 
R. 2305. 
|| PB. M. Duncan, 1872, Mon. Pal. Soc., Part i. p. 14, pl. i. figs. 1-4; B.M. no. 
R. 8455. 
q R. F. Tomes, 1883, Quart. Journ. Geol. Soc. pp. 181, 182, pl. vii. fig. 21; B.M. 
no. R. 8469. 
** H. M. Bernard, 1906, British Museum Catalogue of Madreporarian Corals, 
vol. vi. p. 22. 
