GENERAL AND COMPARATIVE STRUCTURE. 45 



In other cases, in place of proper " monticules," we find other 

 small areas which are occupied by corallites considerably 

 smaller than the average, and which are either level with the 

 general surface, or are even slightly depressed below it (as, for 

 example, in M. frondosa, D'Orb., and M. siLbpulchella, Nich.) 

 Such areas are usually called " macul£e," and they often exhibit 

 a peculiar appearance, seemingly due to the closure of the 

 mouths of their constituent tubes by a delicate calcareous 

 membrane. 



IV. Spiniform Corallites, — Another very remarkable super- 

 ficial feature which is specially common in the thick-walled 

 species, though occasionally present in the thin-walled types, 

 and which is also generally present in the genus Stenopora, 

 Lonsd., is the existence of peculiar blunt spine-like structures, 

 which are placed, in greater or less numbers, round the calices, 

 usually at the angles of union of the corallites. Various Mon- 

 ticuliporoid Palaeozoic corals have been noticed by different 

 observers to possess these calicine spines ; and Milne-Edwards 

 and Haime at one time (Brit. Foss. Cor. Intr., p. Ixi) regarded 

 the existence of these structures as diagnostic of the genus 

 Stejtopora as defined by them. Structures of this nature are, 

 however, possessed by a large number of true Monticuliporce, 

 and notably by M. frondosa, D'Orb., M. tumida, Phill., M. 

 Jamcsi, Nich., AI. vwiiiliformis, Nich. (fig. 5), M. implicata, 

 Ulrich (fig. 4), M. gracilis, James, and other forms. As viewed 

 from the surface, these spines present themselves simply as so 

 many blunt projections, which do not seem, so far as I have 

 been able to observe, to be ordinarily perforated by any apical 

 apertures. I have however, in some cases, detected minute 

 circular apertures at the summits of these spines ; and it is 

 quite possible that they are really generally present, but that 

 they are filled up by the matrix or by infiltrated calcite. On 

 the other hand, when examined by means of thin sections, 

 these spines are found to be in no way of the nature of mere 

 superficial ornaments, but they extend into the substance of 

 the corallum, between the ordinary corallites, to a depth equal 



