LOWER HELDERBERG ROCKS. 
SOI 
Fig. 11. A specimen of the left valve, in which the proportions of length and height are 
nearly equal. The deep marginal sinuosity of the posterior wing is exaggerated, 
from imperfection in the specimen. 
Fig. 12. The left valve, preserving the form nearly entire ; the upper part of the surface 
being exfoliated so as to obliterate the markings. 
Fig. 13. The specimen is apparently the inner side of the right valve, and is quite flat. 
The broad costae represented are the elevated spaces between the external costae, 
as shown on the inside of the shell. 
Fig. 14. A partial cast of the left valve of this species. 
Geological position and locality. In tlie shaly limestone of the Lower Helderberg 
group : Helderberg mountains, Hudson, Schoharie, etc. 
Several other undescribed species of Avicula* are known to occur in the rocks of 
this group; but up to the time of completing the plates (1857), no well-preserved 
specimens had been obtained. The numerous localities of this group of strata along 
the Hudson river from Rondout to the Helderberg mountains, and thence westward 
along the northern outcrops as far as Herkimer county, afford ample opportunities 
for increasing the number of species, not only of Avicula , but of all the Lamelli- 
branchiata. The obtaining of these forms, however, is somewhat more difficult than 
of the Brachiopoda; and as more especial attention has been given to the collecting 
of the latter class of fossils, it is probable that, when compared with those described 
in this volume, a larger proportion of Lamellibrancliiata will be discovered than 
of Brachiopoda. 
This group, like all the others, presents a greater proportion of lamellibranchiate 
molluscs in its more eastern localities; while as they diminish to the westward, the 
Brachiopoda increase in numbers. 
* I have been unable to obtain a specimen corresponding to the Jlvicula rugosa of Vanuxem ( Report, 
p. 112, f. 2 ), and this species is therefore omitted in my list of those belonging to this group. 
