576 NEW YORK STATE MUSEUM 



unsafe, or even wrong correlations, but also gives a wrong conception of the 

 development of the organic Avorld, namely that of a great monotony, by which 

 paleontology would not be enabled to make us acquainted with wholly new 

 groups of forms. 



Wiman also strongly urges that the graptolites can not be brought 

 under any of the now existing classes of animals. To the placing of the 

 graptolites with the Hydroidea, this author objects on the ground that first 

 there is no hydroid known which has the structure of a graptolite, and 

 secondly the persons of the first order of the graptolites (the thecae) are 

 bilateral symmetric, while those of the Hydroidea possess a higher 

 symmetry. 



The possible objection that hydroid thecae also possess a bilateral 

 symmetry is met by the statement that this can be explained by the position 

 of the individuals, while in the graptolites the sicula already possessed a 

 bilateral symmetry. But it is not to be overlooked here that one of the 

 principal features which constitute the bilateral symmetry of the sicula, viz the 

 position of the virgula within one of the Avails, is clearly a later acquisition of 

 the colony, for the earlier forms have no solid virgula, and it has hence by 

 acceleration of development been transferred to the sicula. In a rapidly 

 developing group such as the graptolites manifestly were, it is equally 

 possible that the bilateral symmetry of the thecae, originally induced by 

 their position, became so quickly fixed that by tachygenesis it appeared 

 already in the sicula in a manner similar to the appearance of the 

 virgula. 



It is, further, to be considered that the sicula consists of two parts, 

 an apical and an apertural one. The latter, which is the bilateral symmetric 

 one, had clearly the properties and functions of a theca [p.520] ; while the 

 initial or apical part, which lacks growth lines, was the original embryo 

 sheath. The latter, however, shows to my knowledge, a higher symmetr}- 

 than a bilateral one ; and such higher symmetry was hence, probably, also 

 possessed by the embryo. 



