210 



CHAPTER XIV. 



H YD R0Z0A— Continued. 



Sub-class Graptolitoidea. 



The organisms comprised under the head of Graptolitoidea (the 

 Rhabdophora of Allman) are commonly known as " Graptolites," 

 and constitute a remarkable assemblage of extinct Hydrozoa, all the 

 known forms of which are confined to the Upper Cambrian, Ordo- 

 vician, and Silurian deposits. The Graptolites may be defined (if 

 the abnormal genus Corynoides be excluded) as being composite 

 Hydrozoa in which the organism consists of numerous polypites 

 united by a ccenosarc ; the latter being enclosed in a tubular, chitin- 

 ous polypary, usually forming a complete external membrane of con- 

 siderable strength, while the former were protected within " hydro- 

 thecse." The polypary may be undivided or branched, and was 

 invariably free, the organism apparently floating in the water with 

 the proximal end of the colony directed upwards. In most cases, the 

 polypary is strengthened by a peculiar rod-like axis (the "virgula"), 

 which lies in a groove on the dorsal side of the polypary (i.e., on the 

 side opposite to that on which the hydrothecae are developed), and 

 may be prolonged beyond one or both ends of the colony. The 

 polypary is, typically if not universally, furnished at its proximal end 

 with a minute triangular or dagger-shaped body (the "sicula"), 

 which represents the embryonic skeleton. 



As regards its development, the earliest condition in which a 

 typical Graptolite presents itself is that of a small triangular corneous 

 body, which has been termed by Professor Lapworth the " sicula " 

 (fig. 92, a), and which is the starting-point of the entire polypary. 

 In its earliest stage (fig. 92, a a) the "sicula" is merely a triangular 

 horny sheath lodging the coenosarc, but a rod-like axis or " virgula " 

 (fig. 92, a b) is soon developed along its entire length, often pro- 

 jecting freely at one or both extremities. In the next stage (fig. 92, 



