RELATIONSHIP OF DENDEITINA TO PEXEEOPLIS. 



9 



of Dendritina, moreover, is peculiar ; for, so far as I am aware, this type is restricted to 

 the tropical ocean. I have not met with it in dredgings from any part of the Mediter- 

 ranean or the Red Sea, where Peneroplis abounds ; while the largest specimens I have 

 seen are those contained in Mr. CUMING'S Philippine explorations, some of these measuring 

 078 inch in diameter, and - 030 in thickness or breadth. To such as content themselves 

 with glancing at strongly-marked examples of this type, the propriety of its generic, or 

 at any rate of its specific separation from Peneroplis, would seem indubitable. Never- 

 theless I think that I shall be able to show adequate grounds for the belief, that the two 

 forms cannot be separated by any definite line of demarcation ; and that they must there- 

 fore be ranked as belonging not merely to the same genus, but even to the same species. 



133. In the first place I would refer to the fact that the peculiar plication and puncta- 

 tion of the surface of the shell, which are such marked features in the physiognomy 

 of Peneroplis, are repeated in Dendritina (Plate II. figs. 21, 22) in a manner so precisely 

 similar, as strongly to impress every one who has his attention directed to the aspects of 

 these two forms respectively, with the idea of their very close relationship. 



134. Secondly, the differences of general configuration between Peneroplis and Den- 

 dritina are differences of degree, and present themselves in very variable amount in 

 different individuals. For, starting from those forms of Peneroplis in which the spire 

 is least compressed, the transition is easy to those Dendritina whose spire is least turgid, 

 and whose septal plane is not broader in proportion to its length than it often is in 

 Peneroplis. From the most compressed forms of Dendritina to those which have the 

 most turgid spires and the widest septal planes, the gradation is insensible, scarcely any 

 two individuals according in their proportions ; thus we find that whilst the septal plane 

 is sagittate in some (Fig. II. A, c), it tends to become reniform in others (B, D), the 



Fig. II. 



Front views of four specimens of Dendritina. 



margin of the spire, which is almost carinated in the first case, becoming obtuse and 

 even cresentic hi the second. Again, the extent of the investment of the earlier whorls 

 MDCCCXLIX. c 



