450 Young — On the Genm Heteropkyllia. 



section varies with the prominence of the costae and the convexity or 

 concavity of the intercostal spaces, and this variation may sometimes 

 be seen upon the same specimen. In general, the greatest number of 

 specimens of all the various diameters met with, approach the hexa- 

 gonal form ; one specimen, however, in my collection shows, in a 

 cross section, a stellate or six-sided angular form, and another well 

 marked fragment is quadrangular in section, having four small tuber- 

 culated costaa. Internally this specimen has only four septa corres- 

 ponding with the costae, instead of six, the normal number. Other 

 specimens occur which show a nearly circular section, and on some 

 of these, instead of costae, we have only longitudinal grooves to the 

 floor of which the booklets were fixed. These differences in the ex- 

 ternal form of the stem of the corallum I consider to be mere ex- 

 ceptional variations of one species. 



The endotheca and dissepiments are also characters that vary. In 

 many specimens the endotheca has either never been well developed 

 or has been destroyed by the crystallization of the internal structure 

 of the corallum. The dissepiments are stated to be wide apart, but 

 one longitudinal section shows 30 in the length of one inch, being 

 less than one half line apart. 



It will be seen from the above remarks that this coral varies, to a 

 certain extent, both in its external form and internal structure, many 

 of its parts depending upon the state of preservation in which we 

 find the specimens. It seems to me that Dr. Duncan has taken the 

 specific characters of H. Lyelli from the lower portion of a stem with 

 the booklets broken off and slightly worn, and his H. mirdbilis from 

 the upper or more slender part of a stem, as seen lying in the shale, 

 with the booklets in position. 



There is one important mistake which Dr. Duncan has com- 

 mitted in his description of H. mirdbilis, to which I wish shortly to 

 refer. He states that the curved hook-shaped spines or processes, 

 which stand out from the costae, were articulated to the tubercles 

 upon the costae, and he gives several figures to illustrate what he 

 supposes was their mode of attachment. This view is not warranted 

 by an examination of several fine specimens in my own collection 

 and in that of Mr. James Armstrong, which are embedded in shale, 

 and show the booklets in position. 



These were not booklets articulated upon tubercles, but small, 

 curved, spinous processes immoveably attached to the stem, either 

 upon the costae or in grooves. At their base these processes seem to 

 have been tubular, and when broken off and a little worn, as seen in 

 weathered specimens, they then present a deceptive appearance as of 

 a small rounded tubercle, with a pit in the centre, which is caused 

 by the hollow base of the spine. 



In nearly every specimen in which these delicate little booklets 

 are preserved in position, they are seen to be fractured close to their 

 attachment with the stem. This has been produced by the pressure 

 to which the corallum was subjected while lying in the soft shale. 

 It is easily seen from the irregular way in which they are fractured, 

 that they were broken off by pressure, and not by any process of 



