HISTORICAL INTRODUCTION. 3 



of canals. Thus each concentric stratum, or " latilamina," is traversed by irre- 

 gular vertical canals, which are sometimes crossed by delicate cross-partitions or 

 " tabula?," and there are also numerous irregular and tortuous horizontal channels 

 by which the vertical tubes are placed in communication with one another. 



Though the skeletal elements are thus theoretically divisible into two series, 

 the two are really fused with one another into a continuous reticulation. The 

 general tissue of the skeleton is, therefore, exceedingly similar to that of the 

 recent genus Millepora, Lam., the principal difference between the skeleton of 

 Stromatopora concentrica, Goldf., and that of Millepora, being that the zooidal 

 tubes of the former are not divisible, as they are in the latter, into a series of large 

 " gastropores " and a series of smaller " dactylopores." 



I shall more fully describe and figure the minute structure of the skeleton in 8. 

 concentrica, Goldf., at a later period. It will be evident from the above, however, 

 that the genus Stromatopora, Goldf., as typified by the first-described species, viz. 

 8. concentrica, Goldf., comprises fossils of an entirely different structure to those 

 which pala3ontologists have hitherto usually included under this generic name. I 

 shall be able to show that 8. concentrica, Goldf., is really only one of a very 

 extensive series of forms, abounding in the Silurian and Devonian Rocks, and 

 constituting a well-marked group, for which the name of Stromatojaoridce may be 

 employed. 



On the other hand, the various fossils which have been placed by palaaonto- 

 logists generally under the head of Stromatopora — when this name has been 

 used in a restricted sense — are really of a very different structure, and must be 

 placed under new generic titles. The most characteristic of these, namely the 

 forms understood formerly by Bargatzky, Carter, and others who have specially 

 investigated the subject, as Stromatopora?,, may be included under the new genus 

 Actinostroma, which will form the type of the group of the Actinostromidm. It 

 follows, further, that whenever in the present work Stromatopora concentrica, 

 Goldf., is mentioned, the type understood under this name is the original of 

 Goldfuss, as above described, and is therefore neither generically referable to what 

 has been previously understood as Stromatopora, nor specifically identical with 

 the forms which have usually been regarded as constituting 8. concentrica, Goldf. 



Before leaving the subject of the nature of the original specimens of Stromato- 

 pora concentrica, Goldf., I may mention that the Bonn Museum contains a third 

 specimen, which is believed to have been in Goldfuss's view when he described 

 this species. Being unfigured, this specimen has, of course, no authoritv as 

 compared with the figured specimen above described, which we must take as the 

 real type of S. concentrica, Goldf. It is worth noting, however, that the specimen 

 here alluded to, though in its general aspect and superficial characters very like 

 the true S. concentrica, is in reality of a totally different structure. It has the 



