14 PALEONTOLOGY OF NEW YORK. 



Their bases are almost always more or less curved, sometimes making nearly 

 an entire volution before diverging to the flexuous or straight line. 



On carefully examining the surfaces of shells where these parasitic tubes oc- 

 cur, there are often found numerous circular specs attached to the shell-surface. 

 Many of these, under the naked eye, or an ordinary magnifier, appear as simple 

 circular disc- of calcareous matter, while under a higher power a minute con- 

 volute tube is distinctly visible. These in their larger growth become spiror- 

 biform, and are apparently identical with those described as Spirorbis inornatus, 

 Hall (Twenty-eighth Annual Report of the State Museum of Natural History, 

 page L81, pi. xxxi, figs. 14, 15). 



The material in our possession is not sufficient to demonstrate a gradual and 

 absolute transition from these minute convolute discs, through all the phases of 

 development, to the recognized Cornulites ; but I believe they are identical, 

 and that by careful examination of larger collections all intermediate stages of 

 growth and development may be found. 



The ordinary condition of the young of this species is that represented in 

 figures 1-4, plate cxvi, where the tubes grow singly or in groups, with one side 

 attached and the free portion sharply annulated. In this condition the aper- 

 tures present an irregular margin, as if the continuation of the tube had been 

 broken away, and much the greater proportion of the specimens are found in 

 similar condition. In specimens represented in figures 5 and 6, the acute apex 

 is preserved in its incurved condition. In figure 5 the annulations are less 

 distinct than usual on the earlier portions of the tube, while in figure 6 this 

 feature is strongly marked in the lower part of the specimen, becoming obso- 

 lete above, or merged into the thickening tissue as growth progresses. In 

 figure 7 the apex is slightly incurved, the annulations distinct below, but 

 becoming obsolete above. In figure 8 we have two individuals attached by 

 their bases to the surface of Platyostoma Niagarense. The apical portion is less 

 distinctly annulated than in some other specimens, and the original initial point 

 has probably been dissolved or eroded. In the progress of growth the regular 

 annulations soon become obsolete in the thickened tissue above, which is finely 

 striated longitudinally, and presents an irregular lamellose surface. In figure 9 



