36 PERCY SLADEN TRUST EXPEDITION 



them ; I have not made out any " wall-lamellaj" between the perijDheral parts of the septa 

 (see 108, p. 159). On the other hand, such lamellae, i.e. interseptal tangential bridges 

 with dark lines, are seen in the transverse sections of the coralla of certain species of 

 Favia, e.g. F. doreyensis and abdita, wdiile they are absent in allied species. It would 

 appear from these observations that no great importance could be given to the dark-centres 

 and lines. 



Summary of characters. The main characters that I have made use of in the 

 determination of genera and species, so far as the polyps are concerned, are as 

 follows : 



I. Oral-disc and Edge-zone. (1) The nature and relative thickness of the ectoderm 

 and endoderm. 



II. Tentacles. (2) The presence or absence of either entocoelic or exocoelic tentacles 

 or of both. 



(3) The presence or absence of sub-terminal batteries ; when present, the number 

 of the batteries constituting a longitudinal row. 



(4) The nature of the ectoderm and endoderm. 



III. Stomodceum. (5) The nature of the ectodermal ridges at the attachments of 

 the primary mesenteries — their relative breadth and thickness. 



IV. Mesenteries. (6) In Group I, the number of mesenterial cycles and of the 

 couples constituting each cycle ; in Group II, the approximate number of couples in the 

 principal and the subsidiary cycles. 



(7) The shape, relative size and distribution of the entoccBlic mesogleeal pleats, the 

 presence or absence of exocoelic pleats, and the relative thickness of the mesenterial 

 mesoglsea. 



(8) The nature and relative thickness of the mesenterial endoderm. 



(9) The extent to which the margins of mesenteries are convoluted. 



V. Nematocysts. (10) The nature, distribution and relative abundance of the types 

 present. 



If the reliability of the above characters should be doubted on the ground of their 

 dependence on the degree of contraction, preservation, etc., of the polypal tissues, it is 

 only necessary to refer to the case of species like Goniastrea retiformis (Lam.) and 

 leptastrea roissyana (Ed. and H. ) whose specific characters have remained constant in spite 

 of their wide distribution. There was abundant material of G. retiformis from the 

 Maldives, Funafuti, Fiji and Ceylon, of L. 7'oissyana from the Red Sea and Ceylon, and 

 of Favia abdita from the Maldives and Ceylon, collected by different persons at different 

 times and perhaps also treated differently. In either case the transverse sections of the 

 polyps from these remote localities presented the same appearance, the specific characters 

 being easily recognisable from any one of them. In the case of species with limited 

 distribution, the constancy of the specific characters is maintained in polyps taken from 

 difierent colonies. 



Since the condition of the polypal tissues can be easily judged from the sections, 

 the characters of well-preserved polyps may be relied on as normal. Under every species 



