770 BULLETIN 106, UNITED STATES NATIONAL MUSEUM. 



Measurements- 



Distance between the fascicles 0.17 mm. 



Width of the fascicles 0.11 mm. 



Diameter of the peristomes 0.12 mm. 



Maximum zoarial width 1.8 mm. 



Affinities. This species much resembles Retecava, in the arrangement of its 

 pores, but differs in the nature of its ovicell, which is entirely frontal and not 

 located on a single side of the zoarium. 



The knowledge of the nature of the pores and of better ovicells will perhaps 

 permit of the discovery some day of the true position of this species. 



This species differs from Idmonea svbcancellata Manzoni, 1877. in its fascicles, 

 which are much more salient and formed of more than three to four tubes. 



Occurrence. Lower Jacksonian: Three and one-half miles southeast of Shell 

 Bluff post office, Georgia (rare.) 



Cotypes.C&t. No. 65341, U.S.N.M. 



Genus IDMONEA Lamouroux, 1821. 



1821. Idmonea LAMCWROUX, Exposition Methodique des Polypiers, p. 80. 



The. ovicell is irregular, little lobed or not at all, subsymmetrical, placed on 

 the median crest of the zoarium. The fascicles are not entirely enveloped by the 

 ovicell. The oeciostome is excentric, nonterminal, and replaces the first or the 

 second tube of a fascicle. The basal lamella is simple and bears no pores. 



Accepted genotype. Idmonea atlantica Johnson. 1847. 



Range. Senonian-Recent. 



This genus differs from Tubulipora Lamarck, 1816, in the absence of well- 

 developed lobes, and in its oeciostome nonsubcentral, and occupying a fixed place 

 among the fascicles. 



Historical. The genus Idmonea was founded by Lamouroux upon I . triquetra, 

 an incrusting form found in the Bathonian at Ranville, France. The history of 

 the genus has been given many times, but the greatest confusion has existed among 

 paleontologists as to its interpretation. Among the zoologists, however, the name 

 Idmonea has always been applied to the well-known zoarial form bearing an 

 anterior ovicell. Former authors were ignorant of the importance of the ovicell 

 and even of its existence. All speculation on their work is absolutely useless and 

 idle. Usage and the principle of least change seems to us the only considerations 

 to be regarded, and we have therefore followed the zoologist in regarding Idmonea 

 atlantica Johnston, 1847, as the accepted genotype. 



Determination. The, determination of species of Idmone.a is very difficult in 

 spite of the introduction, due to Pergens, of the micrometric dimensions; this is 

 in reality because we have only isolated fragments for study. If the ovicell in 

 rapport with the number of larvae and the fissiparity of the primary embryo is 

 quite variable in form and position, the oeciostome is more constant, for it is only 

 in rapport with the form and size of the larvae. But the larvae of the different 

 families of Cyclostomata are much alike. It is necessary, therefore, to seek another 



