1 8 INTRODUCTION. 



of the zocecia have the normal arrangement of rings in some 

 parts of the zoarium, while elsewhere they occur in alternate 

 lines as in Idmonea. This type was made by D'Orbigny into a 

 genus, with the name of Bisidmonea, which has been accepted 

 by some later authors such as Walford. But Tubigera antiqua, 

 D'Orb., differs from Bisidmonea only by having an oval instead 

 of a tetragonal section ; and Pergens includes that species among 

 the erect forms of Idmonea. The adnate forms of Idmonea are 

 also brought within the same group by variation along another 

 line from the same starting-point Stomatopora. The only definite 

 distinction between Proboscina and the true adnate Idmonea, is 

 that in the latter the apertures are arranged in regular alternate 

 lines on either side of the zoarium. But such a species as 

 Idmonea virgula, D'Orb., 1 is exactly intermediate between Proboscina 

 and Idmonea. So close do these genera run to one another, that 

 D'Orbigny named a series of species Idmonea on his plates and 

 Proboscina in his text. 



Hence, if the series Stomatopora, Proboscina, Berenicea, and 

 Diastopora are to be united into two genera, we are equally bound 

 to unite them into one, and also to include with this Spiropora, 

 JZntalophora, Bisidmonea, and both the adnate and erect Idmonea, as 

 well as various massive, intermediate types such as Reptomultisparsa. 

 Practically, that is to say, we unite the forms which Busk split up 

 into the three families IdmoniidaB, Tubuliporidse, and Diastoporidse, 

 into a single genus. 



In the other groups of Cyclostomata similar transitions can be 

 traced, and the whole order could be reduced to some half-dozen 

 genera. Any such changes in nomenclature would be fatal to 

 progress in the classification of the group. Great disturbance 

 would be produced in the specific terminology ; for the same 

 names are frequently used in different genera, and will have to 

 be changed if the genera be merged. 



3. Are there any Genera in Cyclostomata^ It may be objected 

 that to accept these transitional forms as genera is indefensible; 

 for we are not justified in abandoning sound methods of nomen- 

 clature, either because the results are inconvenient, or because 



D'Orbigny. Pal. fran9 Terr. cret. t. v. pi. DCXXXI. figs. 15-17. 



