Polyzoa and Antliozoa from Upper Cretaceous Limestone. 5 



CEISINA CENOMANA, d'Orbigny. 



Plate I., fig. 5. 

 1851. Crisina cenomana, d'Orbigny, Bry. Cret., p. 732, pi. 614, 



figs. 1-5. 

 1845. Idmonea disticha, Michelin, Icon. Zooph., p. 204, pi. Hi., 



fig. 18. 

 1853. Idmonea calypso, d'Orbigny, Bry. Cret., p. 733, pi. 747, 



figs. 10-14. 



1890. Idmonea cenomana, Pergens, Eevision des Bry. Cret. fig. par 

 d'Orbigny, p. 344, pi. xii., fig. 3. 



Zoarium subcylindrical, and the reverse face is convex or flat ; 

 there is no keel on the obverse face ; the zooecia are immersed, and 

 when the reverse face is flat the zocecial boundaries are clearly 

 marked ; the lateral series of apertures are quite regular, and just 

 reach the middle line of the obverse face ; there are about four 

 apertures in each lateral series ; the apertures are circular, all of 

 one size ; those of the same series are distant less than one diameter 

 of an aperture ; those of different series two or three diameters of an 

 aperture. 



This definition distinguishes C. cenomana from all cretaceous 

 forms of Eetecava as well as from others of Crisina. Those species 

 of Retecava which most resemble it have a very slight median keel 

 on the obverse face, except E. Abbotti (Gabb and Horn), (Journ. 

 Acad. Nat. Sci. Philadelphia, ser. ii., vol. iv., 1860, p. 404, pi. Ixix., 

 figs. 45-47), but in this form there is a ridge down the obverse face 

 of each zooecium. 



Two distinct forms are included under the above definition, and 

 both occur in the material here described. In the first, resembling 

 d'Orbigny's figure of C. cenomana, the zooecial boundaries are hardly 

 visible and the reverse face is convex. In the second (pi. xlv., fig. 4), 

 resembling d'Orbigny's figure of Idmonea calypso, the zooecial boun- 

 daries are strongly marked and the reverse face is flat. The only 

 justification for placing the two forms under one name is their 

 identification by Pergens.* Speaking of C. cenomana, under the 

 synonyms of which he had placed I. calypso, Pergens says, 

 " quelquefois les branches sont aplaties a la partie inferieure et 

 forment ainsi une variete decrite comme 1. calypso." That is, 

 I. calypso is an earlier growth stage of C. cenomana confined to the 

 proximal portions of the zoarium. In view of the very fragmentary 

 character of the specimens usually obtained of these erect Idmoneids, 

 * E. Pevgens, Eevision des Bry. Cret. fig. par d'Orbigny, 1890, p. 344. 



