534 MM. NicJwhon <^ R. Etheridge, jun, — Solenopora compacta. 



all specimens alike not only show no traces whatever of any concen- 

 tric clear bands, or of any interruptions in the walls of the tubes ; 

 but, so far we can judge, exhibit appearances absolutely incompatible 

 with anything except the cross- sections of tubes. 



Upon the whole, therefore, while admitting our inability to 

 adequately explain the peculiar appearances above described as 

 seen in long sections of many examples of Solenopora, we do not 

 think that these appearances afford sufficient evidence for concluding 

 that the true structure of the fossil is cellular. If evidence can be 

 obtained proving decisively the existence of a cellular structure 

 in Solenopora, then the reference of the genus to the calcareous 

 AlgEe would follow as a matter of course. In the absence of such 

 evidence, we can only leave the question of the affinities of the 

 genus quite open. 



The only remaining point upon which we may say a few words 

 is the geological range of Solenopora compacta ; the species being 

 remarkable not only for its wide distribution, but also for the con- 

 stancy with which it presents itself at a particular horizon. In 

 North America it occurs, apparently in great abundance, in certain 

 parts of the Trenton and Black Eiver Limestones. In Scotland, 

 it is found in the Ordovician limestones of Craighead, near Girvan, 

 which we have elsewhere paralleled with the upper portion of 

 the Trenton Limestone (Mon. Sil. Foss. Girvan, p. 95). In 

 Eussia it was obtained by Dr. Dybowski from Herrkiill, in beds 

 of Ordovician age (" Borckholm beds" of Friedrich Schmidt). It 

 has, however, recently been collected by one of us, in great quantitj'^, 

 in still lower beds in Esthonia, namely in the limestones of Saak, 

 south of Eeval. These limestones are placed by Magister Schmidt 

 in the upper portion of the " Jewesche Schichten," and correspond, 

 therefore, with the Trenton Limestone of North America. At this 

 locality, Solenopora compacta not only occurs as detached specimens 

 of all sizes, but it also makes up almost entire beds of limestone, 

 most of the examples being in this case of comparatively small 

 dimensions. Indeed, some of the bands of limestone at Saak look 

 like amygdaloidal lavas ; while others have a cellular appearance, 

 from the dissolution out of them of the little pea-like skeletons 

 of this fossil. 



Apart, therefore, from the question of its affinities, Solenopora 

 compacta must be regarded as remarkable both for its very wide 

 geographical range, and also on account of its being highly charac- 

 teristic of a definite geological horizon. As before mentioned, 

 however, there are certain differences which separate the Scotch 

 examples of the species from those found in other regions. The 

 type of the species must be held to be the Canadian examples, since 

 it was upon these that Billings founded his Stromatopora compacta. 

 With the Canadian examples, the Eussian examples appear to agree 

 in every particular. On the other hand, the tubes in the Ayrshire 

 specimens are decidedly, and apparently constantly, larger than those 

 of the Canadian and Eussian examples, their general structure being 

 otherwise the same. This difference will be seen by comparing the 



