68 W. Carruthers — On the British Graptolites, 



from the leading modifications in their structure. It is important to 

 us to find that these orders have marked distinctions in the protective 

 coverings of the coenosarc, and it will enable us to establish a more 

 satisfactory comparison if we give the characters of these orders, as 

 far as they bear upon the parts in the graptolites with which we have 

 to deal. 



I. HydridoB. — Hydrosoma consists of a single polypite. The ecto- 

 derm developes no hard cuticular layer. Hydra is the only genus of 

 this order. 



II. Coryntdcs. — Hydrosoma developed into a coenosarc of very 

 various forms, supporting many polypites without thecse. The ecto- 

 derm developes a strong cuticle. The hydrosoma is fixed by a hydro- 

 rhiza. 



III. Sertulariadce. — Coenosarc with a strong, chitinous cuticular 

 layer, which is usually branched, and supports polypites enveloped 

 in thec93. The hydrosoma is fixed by a hydrorhiza. 



IV. Cahjcophoridce. — Coenosarc unbranched, flexible, and contractile, 

 and has no hard, chitinous, cuticular layer. The hydrosoma is free, 

 and the polypites have no thecas. 



V. Physophoridce. — Coenosarc unbranched or very slightly branched, 

 flexible, and contractile, and has no hard chitinous outer layer. The 

 hydrosoma is free, and the polypites have no thecae. 



VI. Lucernariadae. — The base of the hydrosoma is developed into 

 an umbrella. The coenosarc has no chitinous layer. 



There are no parts in the first or in the last three orders which 

 can be compared with graptolites. The members of these four orders 

 are all devoid of any hard cuticular layer. They must therefore be 

 at once set aside, and we are limited to the second and third orders. 

 The Corynidce differ from the Sertulariadce in having no thecae for 

 their polypites ; but the graptolites were certainly furnished with true 

 thecal. In Bastrites the thecae rise at distant intervals from the com- 

 mon canal, and in some species of Graptolithiis they are seen to be 

 distinct structures from the epiderm of the canal. The most obvious 

 indications of these that I have seen are in G. Hislngeri, in which 

 there is a superficial line precisely agreeing with that which is seen 

 at the junction of the theca and the epiderm of the canal in Cam- 

 panular^a, etc. This is figured by M'Coy in his " British Palaeozoic 

 Fossils" (Tab. LB. fig. 7a), and described by him as a septum ; but 

 a specimen in my possession, preserved in the round, shows that the 

 line truly belongs to the outer cuticular layer, and is not an internal 

 structure. This excludes from our consideration the Corynidce, and 

 confines us to the Sertulariadce. In this order there is a hard chi- 

 tinous cuticular layer forming a periderm around the common canal, 

 and distinct chitinous thecae for the individuals of the colony. In 

 these respects no difference exists between the polyzoary of the 

 Ctenostomata and the polypary of the SertuIariadcB ; but the separation 

 between the cell and the common canal by a distinct structure, which 

 is present in all the Polyzoa with a common tube, is absent in the 

 Hydrozoa. In them the wall of the hydrotheca is continuous with 

 the periderm of the common canal, and no septum exists separating 



