94 Dr. H. A. Nicholson on the 



Streptelasma cormculum, Hall, 

 Streptelasma corniculunt, Nicli. op. cit. p. 218. 



Having now made carefully prepared thin sections of typical 

 examples of this the type species of the genus Streptelasma^ 

 I am enabled to give the characters of the genus in greater 

 detail, and to correct one or two errors in my former descrip- 

 tion. The corallum is free, simple, and turbinate, with a thick 

 wall and a well-developed epitheca. The septa (PI. V. fig. 15) 

 are well developed, of two kinds. The majority of the primary 

 septa fall short of the centre of the visceral chamber ; but a 

 certain number are continued inwards, in the form of irregular, 

 tortuous, vertical plates, which often unite with one another, 

 and give rise to a sort of subvesicular axis, which forms a low 

 prominence in the bottom of the calice. Cross sections also 

 show a well-marked septal fossula, including two or three 

 short septa. The secondary septa are short, and alternate 

 regularly with the longer ones. Dissepiments are not absent 

 (as erroneously believed), but are developed to a small extent 

 in a zone between the inner ends of the secondary septa and 

 the central space, into which most of the primary septa do not 

 enter. Some specimens exhibit hardly any dissepiments ; 

 others have a considerable number. In long sections (PI. V. 

 fig. 15 a) the visceral chamber is seen to be traversed by well 

 developed tabulas, which are convex upwards, are elevated 

 centrally, and are somewhat, but loosely, vesicular towards 

 the margin. In sections taken accurately across the centre, 

 there are seen in the median line of the corallum a few ver- 

 tical, sometimes more or less bent or twisted lamellse. These 

 are the edges of the tortuous central plates formed by the in- 

 ward prolongation to the centre of a few of the primary septa. 

 In the transverse section of S. corniculum, Hall, which I 

 have here figured, there are fifty-seven primary septa and an 

 equal number of secondary septa, and three of the primary 

 septa are shorter than the others and stand in the septal 

 fossula. 



The genus Streptelasma^ as founded upon the type species 

 ^S*. corniculum^ can be certainly asserted to be a Rugose coral, 

 and to be nearly allied to Zaphrentis. It differs from Zaphrentis 

 in the smaller development of the tabulae, in the fact that the 

 fossula is not formed by the coalescence of a certain number 

 of the septa, and in the prolongation to the centre of some of the 

 primary septa as so many twisted plates. 



