286 Grajptolites, 



reproduction of the species. The tropliosome consists of a 

 chitinous " polyp ary" that invests the " ccenosarc," or 

 common connecting fleshy basis of the colony, as well as the 

 individual u polypites/'' which are, in some tribes, protected 

 by a specially developed receptacle, or "hydrotheca.'" The 

 c c hydrorhiza " is the root-like proximal termination of the 

 polypary, by which the hydroid is attached to foreign bodies, 

 and the " hydrocaulus" is the portion of the polypary that 

 intervenes between the hydrorhiza and the polypites. In the 

 Sertulariaclce, the " gonophores," or generative buds, are pro- 

 duced in a capsule, called the <c gonangium.-" 



The tropliosome is the only form of the graptolite with 

 which we are acquainted. Its polypary was composed of a 

 flexible chitinous substance, like that of the recent hydroids. 

 It is preserved either as a thin carbonaceous film between the 

 layers of a fissile shale, which, on being split, presents an 

 equally perfect impression on both surfaces, or second in the 

 round in the same shale, when the whole organism is con- 

 verted into iron pyrites, or third in limestone, where the fossil 

 frequently retains its original form ; and while the polypary is 

 converted into a carbonaceous substance, the interior, originally 

 occupied by the ccenosarc and polypites, is filled with the 

 amorphous substance of the rock. I have made several sections 

 of two species (G. priodon, Bronn, and G. Roemeri, Barr.), 

 preserved in the round in limestone from Bohemia, and have 

 also examined specimens of G. Sedgewickii, Portl., and G. con- 

 volutus, His., from the Moffat shales, retaining their natural 

 form ; and though the pyrites is more unmanageable . than the 

 limestone, I have determined that they all agree in struc- 

 ture. M. Barrande has accurately described the structure of 

 the Bohemian species named. G. priodon (Plate, Fig. 1) 

 is composed of a single series of hydrothecae, the walls of 

 which are in conjunction for nearly two-thirds their length, but 

 the outer portion is free. In the longitudinal section (Fig. 15), 

 the relation of the hydrothecas to each other is more obvious. 

 M. Barrande figures the separating walls as double, and 

 though they must be so, I have not been able to detect, even 

 with a high magnifying power, more than a single thin layer. 

 In the process of fosillisation, all traces of the two walls have 

 disappeared in the specimens I have examined, but its double 

 nature is seen when, towards the mouth of the cell, it separates 

 into two portions, the one to form part of the superior hydro- 

 theca, and the other of the inferior. A free space exists 

 between the base of the hydrotheca and the wall of the poly- 

 pary, which was filled with the ccenosarc of the colony. This 

 is the common canal Of M. Barrande. There is no constriction, 

 or septum, at the base of the hydrotheca, separating the indi- 



