C. Lapivortk — On British Graptolites. 503 



in practice to restrict the application of the term tiieca to the exterior 

 and separable portion of the chamber, — in other words, to that which 

 is capable of being broken off from the common portion. There is, 

 however, no constriction or line of separation at the proximal ex- 

 tremity of the theca so limited, but the distinctness of each chamber 

 is clearly visible upon the common portion itself. The natural 

 elevation of the central parts of these chambers within the boun- 

 daries of the common canal is the cause of that peculiar waviness 

 of the dorsal edge of the compressed polypary, which has been 

 noticed in a few monoprionidian species. Indeed, in some forms, 

 this elevation is so pronounced, even in the neighbourhood of the 

 virgula, that the dorsal margin appears toothed or denticulated in 

 the manner of a saw. 



In the duplicate forms the polypary is merely composed of two of 

 these monoprionidian polyparies placed back to back. Their dorsal 

 walls are flattened, and coalesce into a median septum, between the 

 two lamiufe of which, the duplicate virgula (which is apparently 

 formed by the united virgidse of the composing branches) lies im- 

 bedded. The flattening of the dorsal walls is never carried so far 

 as completely to obliterate the distinctness of the common portions 

 of the chambers, but the adherent faces are bent by their mutual 

 appression into a series of waves, answering exactly in number and 

 position to the thecge. The two branches are so fitted together that 

 their thecae constitute a double alternating series in the duplicate 

 polypary. It is this circumstance, in combination with the natural 

 prominence of the central parts of the proximal portions of the 

 chambers, that gives to the suture the remarkable undulation, which, 

 conspicuous in only a very few species, is, in all probability, a struc- 

 tural necessity in the whole of the siculate BiprionidcB. 



The Moffat examples of the genus Glimacograptus of Hall, 

 preserved in a state of relief, place beyond a doubt the duplicate 

 nature of the polypary in this genus, as first pointed out and subse- 

 quently so ably contended for by Professor Nicholson. A very 

 slight pressure applied along the line of the septum in these speci- 

 mens is sufficient to cause the polypary to fall apart longitudinally, 

 and thus expose to view the continuous flattened wal Is, the imbedded 

 virgula, etc., as described by him, while the distinctness of the thecte 

 may be placed beyond question by their being easily broken off 

 the common canal one by one, and individually examined. Com- 

 pressed examples of Climacograptus are sometimes procured, in 

 which the moieties have fallen asunder for a greater or less extent 

 previous to fossilization, and specimens are not infrequent, both in 

 this genus and Diplograptus, in which it seems clearly evident that 

 the composing polyparies must have had diff"erent rates of growth. 



The form Climacograptus typicalis (H.), examined and described 

 by Prof. Hall (Grapt. Quebec. Group, p. 28), may possibly belong 

 to the genus Lasiograptus, to be noticed hereafter. 



In addition to the great section of the siculate Diprionidm, there is 

 a second section, embracing the genus Betiolites and its allies. Such 

 observations as have hitherto been made on the structure of the 



