Development and Mode of Growth of Diplograptus. 245 



The writer has been highly gratified at having placed in his hands, 

 by the courtesy of Prof. James Hall, a large slab bearing a colony 

 of Diplogra2'>tus. The fossil is not only interesting because it 

 adds a new locality and a new horizon — the Hudson Kiver 

 group, the slab coming from a railroad-cut near the Abbey, south 

 of Albany, N. Y. — but also on account of the remarkable size 

 and mode of preservation of the colonial stock. The size of the 

 fossil can be taken from the figure on plate lY which is a repro- 

 duction in approximately natural size. One rhabdosome measures 

 80 mm.; anothei: 70 mm., the latter reaching a lecgth of 40 mm. 

 The. diameter of the whole colony, therefore, may easily have 

 been 200 mm. The slab, however, is covered with broken-off rhab- 

 dosomee of still larger dimensions, and especially with longer 

 hydrocauli. One of these rhabdosomes is 72 mm. long and is 

 attached to a hydrocaulus 67 mm. in length. This example 

 illustrates the remarkable length of the hydrocauli of some 

 rhabdosomes, the bearing of which fact on the question of the 

 mode of life of the colony has been discussed in this paper. A 

 comparison of this rhabdosome with those of the colony makes 

 evident that there was a great difference in the length of the 

 hydrocauli of nearly equally long rhabdosomes ; and the profuse 

 occurrence of detached long-stalked rhabdosomes indicates that 

 the colony, as we see it now, may have lost a great number of 

 them. The latter supposition is supported by the presence of a 

 dense intricate mass of hydrocauli near the center of the colony. 

 It is probable, therefore, that while the remams of 30 rhabdo- 

 somes can be counted now in the colony, the latter, when alive, 

 was composed of a considerably greater number. 



The center is so much obscured by the superposition of several 

 rhabdosomes that it appears as a solid film in which only the 

 outline of the central disc can be discerned with difiiculty. 



A remarkable feature of the colony is the great variety of 

 aspect presented by the rhabdosomes, according to the direction 

 in which they became compressed, and the different states of 

 preservation. Though the majority of the rhabdosomes of this 

 colony, from their general appearance, might be identified as 

 D. pristis, Hall, it would seem that the dimensions of the rhab- 

 dosomes are such as to constitute a difference from the forms of 

 D. pristis, Hall, as found in the Utica slate. Moreover, some of 



