MINUTE STRUCTURE OF STROMATOPORA AND ITS ALLIES. 225 



at this juncture sucb variations and gradations, the more typical 

 forms at all events are indubitably marine organisms often 

 associated with Coral formations, in the concretionary mass and 

 debris of which occasionally sbells, encrinites, and various other 

 foreign bodies are mixed. Their main characteristics are: — 

 Masses laminar, in thicker or thinner layers, concentrically 

 arranged and mostly fastened to a foreign body ; surface with or 

 without elevations, and usually though not always exterior orifices 

 of two kinds ; in some instances channels perforate the substance ; 

 the latter composed of a series of laminae so disposed as to enclose 

 cellular interspaces, rectangular, retiform or otherwise ; stellate 

 surface-tracery and an epitheca are occasionally present. 



In discussing the question as to the affinities and systematic 

 position of the Stromatoporoids, probably the best course to 

 pursue will be to review briefly the chief arguments for and 

 against their allocation to the several zoological groups in which 

 they have been placed by different observers — the groups in 

 question being the Corals, the Hydrozoa, the Foraminifera, 

 the Sponges, and the Polyzoa, — previous to which we will 

 advert to certain marine Plants, viz. the Corallines. 



1. To Nullipores. — Although so far as we are aware no one has 

 suggested Stroma topor a to have a vegetable alliance, nevertheless 

 there is in some respects a wonderful resemblance to certain of 

 the Nullipores. 



Thus many of these Lithophytes or Stone Plants simulate the 

 characters of Stromatopora, inasmuch as they are calcareous in 

 substance, spread or insinuate themselves in thin layers parasiti- 

 cally between corals and other foreign bodies, occasionally form 

 denser incrusting masses, have a nodular and granulated surface, 

 invest and cover objects in successive layers, and in their minute 

 structure present extreme regularity, with a tendency to quadran- 

 gular cell-construction. AVhile these remarks apply in a general 

 way, it by no means follows that the fossil Stromatoporce belong to 

 the group in question ; on the contrary the bulk of evidence goes to 

 support the view that their skeleton is the product of animal 

 organization. 



Setting aside the leafy jointed Corallines as at a glance out- 

 wardly distinctive in form, habit, &c., we were fortunate in ob- 

 taining some large pieces of Nullipores from South Africa (species 

 undetermined) for comparison. These Nullipores were, roughly 

 speaking, of two kinds, viz. a few large crustaceous forms, and 

 others of a short, compact, branching, or interwoven sort. 



