Morpliologrj of the Rhahdophor-ci. 55 



daw and the adjacent hills, amongst which are a few speci- 

 mens from Falcon Crag distinctly showing internal structvire. 

 The species of which the strncture is most clearly defined are 

 Didymogrcqytus extensus, Hall, D. j^ntulus^ Hall, and Tetra- 

 graptus serra, Brong, {=T. hryonoides^ Hall). In several 

 specimens of these species the hydrotheca3 are seen to be sejja- 

 rated from the perisarc by a distinctly-marked septum ; and the 

 perisarc is, moreover, in specimens of all the three species, 

 seen to be jointed, or crossed by transverse septa. 



In a portion of a branch of Tetragraptus serra (tig. 1) this 

 structure is particularly clearly seen. The specimen is pre- 

 served in section with its interior partly tilled in with mineral 

 matter differing altogether from the slaty matrix in which it 

 is enclosed ; and iron-pyrites has taken the place of its once 

 chitinous external membrane. 



On the dorsal margin is the virgula 

 with a wavy outline. Next to this is the 

 jjerisarc or common canal which formed 

 the channel ofcommunication between the 

 individual hydranths, looking where filled 

 in like a jointed tube, and where the 

 infiltrated mineral matter has been re- 

 moved, or has never been deposited, ap- 

 pearing as a series of rectangular depres- 

 sions divided from each other by trans- 

 verse walls or, rather, distinct ridges ; 

 for they do not nearly fill up the space 

 between the two sides of the perisarc. 

 The hydrothecte, where their interior is 

 filled in with mineral matter, are each 

 articulated with the corresponding rect- 

 angular cavity of the perisarc, a ridge 



or partial septum dividing them from it; Fig-. 1. — Part of a 

 and where their interior is not filled in branch of Tetragraptus 

 they are divided from the perisarc and *^'"''^'' uatural size. 

 from each other by a perceptible ridge. 



They are curved, springing from the perisarc at an angle of 

 from 30° to 40°, which gradually increases to 50° ; and they 

 are wider at their distal than at their proximal end, the 

 margin of which is of a curved form, slightly flattened where in 

 contact with the corresponding division of the perisarc. In 

 one portion their external apertures are seen. Here and there 

 the pyrites has filled up spaces which have probably been 

 caused by the contraction of the infiltrated mineral matter, 

 giving a few of the thecas a jointed appearance; but this is 

 evidently an accidental occurrence, and the regular jointing of 



