THE GRAPTOLITES. 363 



this bud thereafter is developed partly the second theca, 

 partly the canal — ' connecting canal ' — which connects both 

 halves of the polypary, and which in the first place gives 

 origin to the third theca (= first theca on opposite side of 

 sicula), and partly also the common canal which connects 

 the second theca with the succeeding ones." He describes 

 the " sicula " which consists of two distinct portions, the 

 "initial part" which he believes to correspond with the 

 original " chitinous covering of the free zooid germ or em- 

 bryo," and the apertural part which has the same function 

 as a theca and may therefore be justly considered as the 

 first theca. Accordingly Holm's second theca corresponds 

 to Tornquist's primordial one, and his third to Tornquist's 

 second. 



The sicula in the bilateral graptolites does not occupy a 

 central position, being partly embraced on one side by the 

 connecting canal, whilst on the other side it is more or less 

 superficial. The sicula side is termed the "anterior," and 

 the other the "posterior". These are used in the same 

 sense as that in which Tornquist employs the terms "ob- 

 verse aspect" and "reverse aspect". The author gives a 

 full account of the connection between the sicula, the first 

 theca, the first bud, from which " arises almost simul- 

 taneously with the left theca the common canal for the 

 left half of the polypary, and the connecting canal which 

 crosses the dorsal side of the sicula and gives origin to the 

 third (or, better, the right) theca lying on the right side of 

 the polypary, and also the common canal for the right side 

 of the polypary," and describes the growth of these in 

 Didymograptus minutus Tornq., D. gracilis Tornq. mut., D. 

 gibberulus Nich., Tetrgraptus Bigsbyi Hall, and Phyllo- 

 graptus angustifo/ius Hall. 



He maintains that a virgula cannot occur in any 

 graptolites of the families Dickograptida, Dictyogr apt idee, 

 and Nemagraptidce, or in the genus Dicellograptus of the 

 family Dic7'anogi r aptid&. The true virgula commences 

 near the apex of the sicula as a prolongation of the same, 

 and corresponds with the thread-like prolongation of the 

 sicula which has long been known in Didymograptus 



