354 PEOCEEDiNGS or THE GEOLOGICAL SOCIETY. [March 8, 



I. South-eastern Arabia. 



Pecten quadricostatus, Sow. 



jequicostatus, Lam. 



Neithea Alpina, B'Orb., sp. 

 Actinospongia, sp. 

 Patellina concava, Carter, sp.* 1 

 Orbitolina concava, Lamk. J 



II. Bagh. 



Neithea Alpina, D' Orb. 

 Pecten quadricostatus, Sow. 

 Khynchonella depressa, 8ow,\ 

 Thamnastrfea decipiens, Michelin, sp. "1 

 Centrastrsea Cenomanensis, Z>' Orb. J 

 Escharina, sp. Vincularia, sp. 

 Eschara, sp. Serpula plexus, Sow. 



The Pecten quadricostatus, Sow., has an immense range, and it is 

 figured by F. Roemer from the Chalk of Texas (Kreideb. von Texas, 

 1852, p. 64). The coral appears to belong to the species which, 

 under these names and that of ThamnastrceapecUculata, De Fromentel, 

 ranged from the Neoeomian to the Hippuritic chalk of Gosau in- 

 clusive. The Upper Greensand horizon of all these forms is suffi- 

 ciently distinct, and is very confirmatory of the geologic position of 

 the Echinodermata. 



6. Description of the Varieties and of a neiv Species. — I. Holec- 

 TTPtrs CiiNOMANENsis, Guerangcr. As a specimen which cannot be 

 distinguished from the type is amongst those from Ras Sharwen, the 

 varieties are easily distinguishable. Dr. Carter considered the species 

 to belong to the genus Discoidea ; but the specimens have not the in- 

 ternal arrangement of that marked genus. He selected the form which 

 is considered by me to be the type of H. Cenomanensis as his " 1st 

 species," and ranged the others under other species recognizable by 

 numerals J. This Holectypus has been ably studied by Cotteau and 

 Triger ; and its abnormal fifth oviductal plate has, after the exami- 

 nation of numerous specimens, been decided to be perforated. All 

 the Arabian specimens show the fifth generative pore. It is inter- 

 esting to observe that the tendency which this species has to vary 

 slightly is seen in the Arabian as well as in the European specimens. 



Variety 1. The specimen is nearly twice as large as the type, has 

 larger tulDercles and less prominent ambulacra! spaces. 



Variety 2. The specimen is slightly larger than the type, and has 

 a pyramidal instead of a rounded upper surface. 



The essential structural details remain the same in both varieties, 

 which are very closely allied. The resemblance of the species to H. 

 macropygus, Agass., sp., of the Neocomiau is very great ; but zoolo- 

 gically the fifth pore necessarily places it in a subgenus ; it does not, 

 however, interfere much with the idea that H. Cenomanensis is a 

 descendant by variation (probably in consequence of obeying some 

 law of "correlation of growth") of the older type H. macropygus. 



There is a species {H.planatus, Roemer) described by F. Roemer§, 

 from Fredericksburg, which is barely specifically diff'erent from that 

 under consideration ; and MM. Cotteau and Triger || , in their admii'able 

 analysis of the siDccies which have come before them, have detected 

 other Holectypi with five generative pores. They decide that H. Tu- 

 ronensis, Defr., sp., of the Chalk of Touraine, extends into the Upper 



* Prof. T. Eupert Jones has kindly determined this species for me. 

 t Mr. Davidson informs me that this Hhynchondla resembles the variety found 

 in the Irish Upper Greensand. X Carter, of, cit. p. 605. 



§ Kreideb. Texas, p. 84. || Echinides de la Sarthe, 1861. 



