28 Reports and Proceedings. 



freely articulated thoracic segments, and three anchylosed abdominal 

 ones and a telson ; 2nd, Prestwichia, a new genus, having the thoracic 

 and abdominal segments anchylosed together; and 3rd, Limulus, 

 Miiller, having a head composed of seven cephalic and one thoracic 

 segments, followed by five coalesced thoracic somites bearing 

 branch!^, and one or more coalesced apodal abdominal somites, 

 to which is articulated the telson. Although so great a dissimilarity 

 exists between Pterygotus and Limulus, yet in the genera Hemiaspis, 

 Exapinurus, and Pseudoniscus, we have forms which, in the number 

 of body-rings, are intermediate. 



The order Merostomata ofi'ers a parallel group to the Decapoda ; 

 the Eurypterida representing the Macrura, and the Xipliosura the 

 Brachyura. The author did not, however, intend by this comparison 

 to indicate that Limulus was higher in the Crustacean scale than 

 Pterygotus, but rather that the former was one of those low, but 

 persistent types, like the BracMopoda, which have remained un- 

 changed through long geological ages, whilst forms capable of 

 further development, like Pterygotus, have been modified and dis- 

 appeared. 



December 5, 1866.— Warington W. Smyth, Esq., M.A., F.K.S., 

 President, in the chair. The following communications were read : — 



1. "A Description of some Echinodermata from the Cretaceous 

 rocks of Sinai." By P. Martin Duncan, M.B., Sec. G.S. 



The existence of Cretaceous rocks in the district of Sinai has been 

 surmised for several years ; but owing to the scarcity of fossils, they 

 have not been correlated with any of the Asiatic formations. An 

 examination of the Echinodermata collected by the Eev. F. W. Hol- 

 land from the limestones of Wady Mokatteb and AVady Badera has 

 enabled Dr. Duncan to show their parallelism with the red lime- 

 stones in South-eastern Arabia, the fossils from which he described 

 in a former paper. All the species not determined are well-known 

 forms, characteristics of the typical Upper Greensand of Europe ; 

 but those formerly described from Sinai by MM. Desor and D'Or- 

 bigny seem to be peculiar to that region. The author observed that 

 by adding the Echinodermata from Sinai to those from South-east 

 Arabia, we obtain a fauna eminently characteristic of the Middle 

 Cretaceous period ; and in conclusion he drew attention to the 

 interesting fact that the majority of the wide- wandering Echino- 

 derms had a tendency to vary from their types both in Europe and 

 in Arabia, while the rest remained persistent in form. 



2. " Geological Description of the First Cataract, Upper Egypt." 

 By J. C. Hawkshaw, Esq., F.G.S. 



At the first cataract the Nile flows over crystalline rocks con- 

 sisting principally of quartz, felspar, and hornblende, combined in 

 various proportions, and then appearing under the forms of syenite, 

 greenstone, hornblende, and mica-schists, or else occurring in sepa- 

 rate masses. In the bed of the river the surface of the harder por- 

 tions of these rocks is beautifully polished. The whole district is 

 traversed by dykes of greenstone, of which the prevailing direction 

 is E. and W. 



