180 Notices of Memoirs — Three Papers on Graptolites. 



the Baltic, has not yet been discovered, and different opinions have 

 been expressed as to the particular divisions of the Silurian to which 

 the boulders belong. By the majority of German geologists they 

 have been referred to the highest series of the Silurian ; the Swedish 

 geologists on the other hand place them about the middle of the 

 Upper Silurian series. With the view of elucidating the question, 

 Dr. Jaekel has examined the Silurian outcrops in the West of 

 England, and be finds that there is a very close correspondence both 

 in the petrological characters and the fossils of these boulders with 

 the beds of Wenlock Shale age exposed at Burrington, near Ludlow, 

 and therefore he maintains that they really are of the age of the 

 Wenlock Shale. 



lu the present paper the characters and the geological distribution 

 of the fossils in the boulders are treated of, but the author more 

 particularly refers to the Graptolites, and brings forward some new 

 structural features which in his opinion will considerably modify 

 their present classification. Thus in the genus Monograptus, two 

 groups are proposed, based chiefly on the different position of the 

 thecal aperture and its appendages. In the first of these, Pristio- 

 graptiis, the aperture is free, and occupies the entire upper end of 

 the theca, and the only appendages are spines on the lower margin 

 of the aperture, and these are not always developed. In the second 

 group, Pomatograptus, the outer portion of the theca is contracted, 

 the aperture is small and situated beneath an extended roof-like 

 process which forms the upper end of the theca. Hitherto the 

 thecal aperture in these Graptolites has been supposed to be 

 at the extreme end of this arched process, but the well-preserved 

 examples figured by Dr. Jaekel show that this view is erroneous. 



The author further maintains that Graptolites were not free- 

 swimming organisms, but probably lived at the bottom of deep seas 

 lightly anchored in the mud. He also considers that the simple 

 forms of Monograptus are not complete, but only branches of colonial 

 stocks ; but, as pointed out by Dr. Holm, he seems to have over- 

 looked the fact of the presence of the sicula, which is never wanting 

 at the proximal end of the organism, and thus conclusively shows 

 the primary commencement of the growth of the polypary. A 

 description of the structure of Betiolites is likewise given, but in this 

 no account is taken of the earlier works of Tornquist and Tullberg. 



In the second paper above mentioned. Dr. G. Holm has revised 

 the list of Graptolites occurring in the Silurian strata of the Isle of 

 Gotland, and enumerates the following nine species and varieties ; 

 Dictyonema cervicorne, n. sp., D. abnorme, n. sp., Monograptus priodon, 

 Bronn, D. priodon, var. Flemingii, Salt., M. subconicns, Tornq., M. 

 dubius, Suess, M. sp., Betiolites Geinitzianus, Barr., and B. nassa, 

 n.sp. A list is given of the names and distribution of the known 

 species of Dicti/onema, and a very careful description of a new species, 

 D. cervicorne, based on specimens obtained free from matrix. In 

 these the upper portion of the theca is extended into a long, spined 

 process bifurcated at the extremity ; and connected laterally with 

 each theca there is a cup-like or nest-shaped structure, possibly a 



