DEVOVIAN FOSSILS. 499 



the imperfect wall is regarded as equivalent to the theca of most rugose 

 corals, the outer wall of Acervularia, while the surrounding tissue is 

 exothecal in nature, a sort of ccenenchyma. If this distinction can be 

 established, the two genera would be widely different in fact, however diffi- 

 cult it would be to distinguish them in practice. However, there seems to 

 be little if any reason why the single, imperfect central walls in both genera 

 should not be homologous with each other and with the imperfect inner 

 wall of Acervularia, Thus the two differentiating characters mentioned by 

 Hall and Whitfield fall to the ground, for in neither genus are the corallites 

 limited by an outer wall. As to the exsert form of the cells, this character 

 is not mentioned in the generic description of Pachyphyllum, but P. 

 bouchardi is described with "Walls strong and distinct; calyces circular, 

 deep, with edges rather elevated." This character, then, is not regarded as 

 of generic value, and in the type species is not especially striking. The 

 chief points of distinction which can be drawn from Edwards and Haime's 

 description (leaving out their view of the homologies of the inner wall, 

 which I hold to be questionable) come to this, that Pachyphyllum has 

 very extended tabulse, while Smithia has them only slightly developed, a 

 character which, taken alone, is of doubtful generic importance. 



Smithia is generally regarded as a synonym for Phillipsastrsea, but 

 Edwards and Haime claim that it is distinguished from the latter by the 

 presence of a columella. If constant, this would seem to be a good 

 character. After studying the type species, Nicholson states that Phillips- 

 astra?a (=: Smithia) has essentially the structure of Heliophyllum — i. e., with 

 carinate septa, fossula, and without an inner wall. If Smithia is indeed 

 synonymous with Phillipsastrgea, then Pachyphyllum is a quite different 

 thing. But if Smithia and Pachyphyllum are as Edwards and Haime have 

 described them, it seems probable that the latter is a synonym of the 

 former. 



It is possible that three types are included among the five species 

 referred to Pachyphyllum. One of these, P. solitarium, I think, beyond 

 a doubt, must be placed elsewhere. Or, is it perhaps the initial cell 

 of a Pachyphyllum colony? Another type which is structurally near 

 Smithia is that represented by P. bouchardi and P. woodmani, A third 

 type is found in P. devoniense and the form above described. It is 



