338 Dr. J. W. Gregory — An Egyptian Milkporoid Coral. 



intimate structure has been completely obliterated. Accordingly 

 it is often quite impossible to come to any final decision as to 

 the affinity of such a fossil ; we have to be content with temporarily 

 assigning it to one of the three groups, according to the balance 

 of probabilities afforded by a series of characters, none of which are 

 of absolute value. 



The collection of Egyptian fossils recently sent for determination 

 to the British Museum by Capt. H. G. Lyons, R.E., includes one 

 interesting fossil which illustrates this difficulty. It comes from 

 the Turouian limestones of Abu Eoasch, near Gizeh. It consists 

 of an irregularly ovoid mass, formed by a thick encrustation round 

 a gastropod, probably a Nerincea} A preliminary external examina- 

 tion left me quite in doubt whether the fossil was a Hydrocoralline, 

 an Alcyonarian, or a Bryozoan allied to Heteropora. Unfortunately 

 a good deal of the interior of the coral has been silicified ; and 

 in the siliceous layers (the dark band of PI. XIII, Fig. 16) the 

 structure has been almost entirely obliterated. Sufficient, however, 

 is left to show that the fossil has the following characters : — 



1. The skeleton, although apparently tubular, is really reticular, 

 being composed of vertical pillars connected by intermediate plates. 



2. The interspaces appear on the surface as a series of pores. 



3. The pores lead down to interspaces which, owing to the 

 connection of the pillars by more or less vertical laminEe, appear 

 tubular. 



4. The pores and apparent tubes are arranged quite irregularly, 

 or in small cyclo-systems, or in linear series separated by branching, 

 radial grooves. 



5. The " tubes " are crossed by tabulge. 



6. The skeleton is traversed by short, broad, flexuous, horizontal 

 canals. 



The reticular skeleton and the canal system both preclude the 

 reference of the fossil to the Bryozoa : this conclusion is supported 

 by the cyclo-systems, which resemble those of the Coelenterata 

 rather than of the Bryozoa. So the fossil is limited either to 

 the Hydrozoa or Anthozoa. It is unnecessary to refer to the 

 characters which differentiate this fossil from most of the sub- 

 divisions of the two groups. If the fossil be an Anthozoan, it is 

 clearly an Alcyonarian allied to Heliopora ; and if an Hydrozoan 

 it is obviously one of the Hydrocorallinse. At first sight the 

 specimen appears to resemble the Alcyonarians, owing to the 

 compactness of its walls and the tubular structure of the whole 

 colony. There is none of the extremely loose, vesicular tissue and 

 abundantly ramified, irregukr canal system of such Hydrocorallines 

 as Millepora and Sporadipora. To illustrate the characters of two 

 typical members of the Alcyonarian and Hydroid corals figures 

 have been given on the plate of sections of Heliopora and Sporadi- 

 pora. Contrasting them we find the following differences : — 



^ Nerinma has been recorded from this locality and horizon by "Walther, 

 " L' Apparition de la Craie aux environs de Pyramides " : Bull. Inst. Egypt, 

 ser. 2, No. 8 (1888), pp. 6, 7. 



