148 Dr. Alex. Broicn — On Solenopora. 



other particular, to the typical species described from Canada 

 and Esthonia, and so he distinguishes the Ayrshire forms as a 

 special variety — Solenopora compacta, var. Peachii. 



Observations. — It is observed that throughout the v^hole of the 

 above species increase takes place for the most part by longitudinal 

 division of the tissue-cells, and that the cells tend to grow in length 

 more than in breath. Not only so, but there seems to exist a 

 species of sub-dichotomy or sub-trichotomy by vphich two or three 

 peripheral cells arise from the outer end of a more central cell 

 (see Fig. 1). This pi'ocess has also been found to occur in certain 

 of the calcareous Algae' (^-g- Ldthotliamnion). Transverse division 

 occurs in other species, and in the description of the following 

 forms it will be seen that there are types, very like Solenopora 

 compacta, in which cell-multiplication by transverse fission is the 

 rule. Even in the present species such a mode of increase is by 

 no means entirely absent. 



Formation and Locality. — Ordovician, Craighead Limestones, 

 Girvan, Ayrshire; Trenton and Black Eiver Limestones, Noith 

 America ; Limestones of Saak, Esthonia, where it makes up great 

 beds of limestone, most of the examples in this case being com- 

 paratively small. Dr. Dybowski also obtained it from Herrkiill, 

 Esthonia, in beds of Ordovician age (" Borckholm beds " of Eriedrich 

 Schmidt). The specimens examined are from the collection of Prof. 

 Nicholson. 



2. Solenopora lithothamnioides, n.sp. (PI. V. Fig. 2.) 



The single specimen examined shows that this species occurs in 

 conical masses of the size of a walnut. Its fi'actured surface has 

 a distinct porcellanous aspect, and exhibits a greyish-brown colour. 

 It possesses the concentrically laminated structure characteristic of 

 the genus. It is much harder and more compact in texture than 

 any of the other species. 



In vertical section distinct concentrically arranged layers of cells 

 are seen. Each layer is composed of numerous cells arranged in 

 parallel rows. The walls of the cells are slightly sinuous, when 

 examined with a high power-. The transverse divisions between the 

 cells are thinner than the vertical divisions, and in many cases are 

 obliterated, leaving fine concentrically disposed clear markings. 

 Cells -i\- — -gig- mm. long and iV — -2^0- rti™. broad. In tangential 

 section the cells are more or less rounded and only slightly sinuous 

 in their outline. The cell-walls are thinner than in S. compacta. 



Observations. — This species is nearly allied to S. compacta, and 

 differs mainly in the character and form of its cell-wall, in the 

 greater regularity of the concentiio layers, and in the fact that each 

 concentric layer is composed of parallel rows of small cells. As 

 regards the last point, the same may be true of S. compacta, if we 

 are to accept the presence of the commonly observable fine con- 

 centric clear spaces as indicative of the original divisions between 



1 A. Eothpletz, "Fossile kalkalgen aus Familien der Codiaceen und der Coral- 

 lineen." Zeitschr. d. Deutscheu geolog. Gesellschaft, Jahrg. 1891. 



