SPONGES, GRAPTOLITES, CORALS. 



Aulopora. 1 



? Order ALOYONARIA. 



Family AULOPORIDJE. 

 AULOPORA (?) TRENTONENSIS, n. sp. 



PLATE G, FIGS. 26-28. 



Corallum adnate, growing on ramose bryozoa of various species, uniserially dis- 

 posed. In most cases the bryozoan has kept pace with the Aulopora and grown over 

 it, so that the sub-circular apertures of its corallites alone remain to attest its pres- 

 ence. Corallites short, about 2 mm. long, subconical, increasing rapidly to the 

 aperture ; the latter elevated, slightly oblique, subcircular, about 1 mm. in diameter ; 

 outer surface with faint encircling lines, and occasionally still finer longitudinal 

 lines. Generally, every second corallite gives rise to two buds, and these in their 

 turn to one each. The buds are developed from the under side in the anterior 

 third of the parent corallite. In rare instances three buds seem to have been given 

 off simultaneously, but one of these is always abortive, failing to attain full develop- 

 ment. Interior of corallites apparently without structures of any kind, the cavities 

 being filled with the matrix only, and where this is removed, in direct communication 

 with each other throughout the colony. Blunt spine-like projections may be noticed 

 on the inner side of the lower wall, but these occur only where the corallum has 

 grown upon Batostoma winchelli Ulrich, or such other forms having well developed 

 acanthopores, to show through the substance of the parasitic Aulopora. 



The absence of septal striae and spines, and of tabulae throws some doubt upon 

 the generic position of this interesting species, and we are not satisfied that it is an 

 Aulopora. Still, as the coral agrees very well with the genus in all its external 

 characters, and since the internal characters of but few of the species are known, we 

 believe it sufficient for the present needs to classify the Trenton species as above. 



This is the only coral of the nature of Aulopora known to us in Lower Silurian 

 rocks, the A. arachnoidea Hall, being a bryozoan of the genus Stomatopora, Bronn. 

 There is no associated fossil \vith which it might be confounded, with the possible 

 exception of the tubiculous annelid Conchicolites minor Nicholson, the tubes of which, 

 like the present species, frequently attach themselves to monticuliporoids, and were 

 more or less completely overgrown by their host. Even in the latter event, the 

 rounded aperture of the Conchicolites left open in the surface of the bryozoan may 

 be distinguished by their less regular distribution and rather smaller size. 



Formation and locality. Not uncommon in the lower and middle beds of the Trenton shales at 

 Minneapolis, St. Paul, three miles south of Cannon Falls, and near Fountain, Minnesota. 



Collectors. E. O. Ulrich, and the writers. 

 Mut. Reg. No. 8240. 



