TEENTON LIMESTONE. 149 



I 



ACEPHALA OF THE TRENTON LIMESTONE. 

 » Plates XXXIV., XXXV. & XXXVI. 



In the fossils of this class, wc find a much g^roater difliculty in the determination of 

 genera and species, than in the Brachiopoda, where the external characters alone are often 

 sufficient for this purpose. The Acephal.4, also, are usually less perfectly preserved, and 

 separate from the rock with greater difficulty, rarely presenting us with any of those 

 characters on which generic distinctions in such shells are commonly founded. In far the 

 greater number of instances they occur as separated valves imbedded in the stone, and in 

 parts of the rock of such a nature that the shell is partially or entirely exfoliated, obliterating 

 even the means of identifying them by this slender aid. The form of the fossil, therefore, 

 is generally the principal or only means of its identification ; and every palaionlologist 

 knows how liable this is to variation, from pressure, and other causes of which we can 

 have but a slight knowledge. It will not be surprising, therefore, that differences of opinion 

 may exist as to the propriety of the reference of many of these species to recent genera, or 

 to genera constituted to receive analogous fossil species of a later epoch, which have been 

 studied with better advantages. 



Notwithstanding the general paucity of species of this order, I am able to present a much 

 larger nuailjer than have ever before been known from the Lower Silurian period, where 

 they have been regarded as very rare. Among these there are two or three forms which 

 present characters requiring a generic designation, and wliich appear to me not only tiius 

 distinct, but equally distinct from analogous forms in the later deposits. Acting therefore 

 upon the conviction that more good can be accomplished by attempting to discriminate, 

 than to risk confounding with what is really unlike, I have attempted to point out characters 

 which maj' serve at least to distinguish several fossils now generally referred to recent 

 genera, and from which all our knowledge, as well as analogy, would teach us to separate 

 them. 



The Genus Cypricardia has heretofore been the receptacle of a large number of species 

 widely dill'ering from each other, and few, if any, of which probably belong to the genus. 

 The name of Nucula has also been appended to shells, having few or none of the essential 

 characters of that genus. Avicula, in like manner, has been the name applied to many 

 shells possessing a remote analogy to some forms of that genus ; and Inoceramus, a genus 

 of the Oolitic and Cretaceous periods, has received both Silurian and Carboniferous species. 



It is true, as just observed, that in many instances it is almost impossible to ascertain 

 with certainty the essential generic characters of these sliells ; but in such cases we should 

 study with more care the external form, and the structure of the shell, which may enable 

 us to arrange the fossil species in natural groups. 



