38 SIXTEENTH KEPOET ON THE CABINET OP NAT. HISTORY. 



2. OBSERVATIONS UPON SOME OF THE BRACHIOPODA, 



WITH REFERENCE TO THE CHARACTERS OF THE GENERA 



CRYPTONELLA, CENTRONELLA, MERISTELLJ, TREMATOSPIRA, 

 RHYNCHOSPIRA, RETZIA, LEPTOCCELIA, 



AND ALLIED FORMS.* 



In the study of the Palaeozoic Brachiopoda, we are often forced 

 to rely upon the general external form, and texture of the shell, 

 for determination of the generic relations, until more extensive 

 collections may furnish weathered specimens, or crystalline or 

 silicified ones, which, admitting of being cut and macerated in 

 acid, will enable us to ascertain the true interior characters. 



In many instances, so nearly do very distinct genera approach 

 each other in their external form, that reliance on this alone is 

 very uncertain, and will surely lead to much confusion if insisted 

 upon as the means of generic determination. 



For a long time, and until we began to learn something of in- 

 terior structure, a large number of species, now known to belong 

 to distinct genera, were embraced in the designations Terebra- 

 TULA and Atrypa, At a later period, when the Genus Rhyncho- 

 NELLA had been recognized in its application to many palaeozoic 

 forms, we find numerous species, which from external characters 

 had been referred to that genus, possessing characters incompa- 

 tible with it.f Among these, some of the forms which have been 

 placed under the Genus R'etzia are not readly separable from 

 well-marked species of Rhynchonella, as will be seen in referring 

 to the species placed under the former genus. 



So long as we remain unacquainted with the interior of the 

 shell, we are compelled to refer the species to some genus having 

 similar external forms, though the fibrous or punctate texture 

 may in many instances prove a valuable aid in these references. 



Among the forms most difl&cult to determine are the numerous 

 smooth or finely striated terebratuloid shells, having either ovoid, 

 elongate, subcircular, or transverse forms. Among the genera of 



* This article was originally prepared for the Report on the State Cabinet; but a 

 part of it has been already published in the Transactions of the Albany Institute, in 

 February 1863. Some changes have since been introduced into that part, from in- 

 formation subsequently obtained in reference to the interior structure of Centronella. 



t The Genus Rhtnchonella was established in 1809, many years before the Genus 

 Atrypa was proposed ; but the former was, for a long time, not fully recognized in the 

 French and English publications. 



