LOWER HELDERBERG ROCKS. 391 



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 costse, 

 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 the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, Hudson, Schohai-ie, 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 Lamellibranchiata will be discovered than 

 of Brachiopoda. 



Tliis 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 niunbers. 



* I have been unable to obtain a specimen corresponding to the jivicvla rugota of Vanvxex ( Report, 

 p. 112, f. 2 ), and this species is therefore omitted in my list of those belonging to this gronp. 



