Yol. 59.] HETETLASTRJEA RHsETICA. 405 



by the much-embedded specimen, has a somewhat peduncular 

 form, with a spreading and gibbous or lobed upper or calicular 

 surface. There are two portions exposed which are near together, 

 and ma}' be taken as parts of the same corallum. The larger one 

 consists of twenty calices (fig. 1, p. 404) which are well defined ; 

 and the smaller one has eight calices (fig. 2), which scarcely project 

 above the level of the matrix, and exhibit evidence of having been 

 worn down. A portion of the side of the corallum is exposed, 

 showing indications of a common wall and rudimentary costa?, 

 but no epitheca. It bears great resemblance to the peduncular 

 parts of Elysastrcea as figured by Laube. 1 



All the calices are small and irregular, both in size and form, 

 the largest not exceeding 2 lines in diameter, and the smaller 

 being of only half that size. They are more or less lozenge-shaped, 

 and there is a distinct interval observable between two of them, due 

 to the imperfect union of the corallites. Between all the others 

 there is a thick and prominent wall. All the calices are of medium 

 depth. 



In a well-developed calyx there are six systems and three cycles of 

 septa, with a rudimentary fourth. All the septa exhibit the peculi- 

 arity common to several B-hsetic Madreporaria, of being thin at their 

 connection with the wall and becoming thicker as they approach the 

 fossula. Those of the first and second cycles meet and unite in the 

 fossula ; those of the third are three-fourths the length of those of the 

 first ; while the septa of the fourth cycle are irregular in length, as 

 well as in their degree of development. 



The margins of the septa, though somewhat worn, present a rounded 

 outline and are denticulated, the denticulations being few in number, 

 not more than six or seven on the longest septum. There are a few 

 dissepiments which almost assume the character of tabulae. 



Both gemmation and fissiparity are very obvious on the upper 

 surface of the corallum. 



Inches lines. 



Height of the corallum, probably 9 



Greatest diameter of the same, about 1 2 



Diameter of the largest single calyx 2 



Since the definition of Heterastrwa in 1888 2 the genus has been 

 found to extend upward into the Inferior and Great Oolite ; and in 

 all the Oolitic species there is a distinct basal or common wall which 

 sometimes has well-defined costse, but in no instance a trace even of 

 epitheca. In the Liassic species, on which the genus was founded, 

 the wall and its costse are merely rudimentary. 



The elimination of the species of HeterastrcBCt from Isastrcea and 

 Latimceandra reduces the species of those genera materially, and at. 

 the same time renders their definition, hitherto very loose and! 

 unsatisfactory, much more definite and concise. 



1 'Fauna d. Schichten v. St. Cassian ' pt. i (1865) pi. v, fig. 6. (Denkschr. cL 

 k. Akad. d. Wissensch. Wien, vol. xxiv.) 



2 Geol. Mag. 1888, p. 207. 



2f2 



