RELATIONS TO EXTINCT ORGANISMS. 83 



c). In such sections the walls appear to be conspicuously 

 moniliform, belnor composed of fusiform thickened regions, 

 which alternate with comparatively delicate unthickened tracts 

 of the wall. This peculiar structure of the w^all is more or less 

 strikingly characteristic of all the species of Stenopora, Lonsd., 

 with which I am acquainted ; and it constitutes the most 

 marked structural feature, perhaps, by which we may readily 

 separate this genus from JMonticiilipora. There are, how- 

 ever, other characters by which, when they can be detected, 

 Stenopora, Lonsd., can be distinguished from any of the Alonti- 

 culiporoids. The most important of these characters is the 

 presence in Stcnopora of " mural pores," precisely similar in 

 their nature to the openings in the walls which go by this 

 name in Favosites. Some species of Stenopora (as S. yackii, 

 Nich. and Eth. jun., fig. 11, c) show these pores in an un- 

 mistakable form ; but they are always minute in point of size, 

 and remote and Irregularly distributed, and there are various 

 species of the genus In which they have not yet been detected. 

 In spite, therefore, of the fact that these apertures must always 

 be present in the genus, It must be admitted that the difficulty 

 which attends their recognition is so great that they are prac- 

 tically of little use In enabling us to distinguish the species of 

 Stenopora from those of Monticulipora. 



Another feature which is characteristic of Stcnopora Is, that 

 many of the calices may be closed by a concave diaphragm, 

 perforated with a central oval or circular aperture. This 

 feature is well seen In examples of the form which I have 

 named Stcnopora Howsii} and of which I have figured the 



1 Stcnopora Howsii, Nich. — As this species— the only undoubted form o[ Stcno- 

 pora, Lonsd., from British rocks with which I am acquainted — has not been pre- 

 viously described, I append here a brief diagnosis of its characters. Corallum 

 ramose, of large size, fragments being over six inches in length, with a diameter of 

 seven lines. Stems sub-cylindrical, with slight tumid enlargements at intervals, the 

 branches remote, and given off alternately from opposite sides of the main stem. 

 Large calices moderately thin-walled, polygonal, about i-6oth inch in diameter, 

 with a larger or smaller number of intercalated minute tubules, the latter sometimes 

 forming " maculae" or irregular aggregations. Calices sometimes open ; sometimes 

 closed by a concave diaphragm, perforated centrally by an oval or circular aperture. 

 Walls of the tubes periodically thickened, and annulated externally in the outer 



