508 MAYOR. 



Both photograph and drawing show two pairs of very small exo- 

 septa. Owing to ignorance as to which of the median septa is dorsal, 

 it is not possible to say whether the upper pair is the dorso-lateral or 

 ventro-lateral pair. The photographs and drawing have been ori- 

 ented, however, to correspond with the arrangement found by Duer- 

 den (:04) in Siderastrea radians, where the dorsal septa are the first 

 to develop. 



b. Description of Skeleton B. 



In this skeleton a distinct epitheca surrounds the basal plate. 

 The entosepta are relatively larger than those of skeleton A. 

 There is the same bilateral symmetry with regard to the angles 

 which the lateral septa make with the median plane. The lateral 

 septa, however, in this case are slightly concave on the lower side. 

 Exosepta and columella are absent. 



c. Conclusions on the Early Development of the Skeleton. 



The early skeletons obtained show great variation in size and in 

 the development of the septa. In some cases only five septa are 

 developed, in others one or more septa are defective. This variation 

 may be due to the fixation of the larvae at an earlier period in their 

 development than is usual; the skeletons showing most variation are 

 the smaller ones. 



The following conclusions may however be drawn. The basal 

 plate and the six primary entosepta are the first structures to be 

 developed. The primary exosepta do not arise simultaneously. Bi- 

 lateral symmetry is frequently shown in the arrangement of the 

 primary entosepta. 



Four possible explanations of this bilateral symmetry occur to the 

 writer. (1) The polyps which formed these skeletons may have fixed 

 themselves with the dorsal or possibly ventral side bent over toward 

 the substratum. (2) One or more of the median entocoels may have 

 been enlarged by the growth of the wall of the polyp in that region. 

 (3) It may represent a persistence of the bilateral symmetry seen in 

 the development of the mesenteries. (4) It may represent the 

 tendency of the coral to grow upward at one point to form the frond- 

 like corallum of the older coral. 



