22 PALEONTOLOGY 



secreting a shell with dorsal and ventral dissimilar 

 symmetrical valves. 



In the opinion of most students of the phylum, the 

 most fundamental plane of cleavage is that between 

 forms like Lingula and Discinisca, in which the valves are 

 not articulated by teeth and sockets, and those like 

 Terebratula in which they are. These two divisions con- 

 stitute the class Inarticulate and the class Avticulata 

 respectively.* The characters of the delthyrium afford 

 the next means of subdivision. Inarticulata such as 

 Lingula which have no delthyrium constitute the order 

 A tremata ; those with a delthyrium enclosed by the 

 ventral valve (at least during early life), the Neotremata ; 

 Articulata with a median deltidium form the order 

 Protremata ; those with a pair of deltidial plates, Telo- 

 tvemata. Each of these orders is divided into sub- 

 Orders (or, as Prof. Schuchert prefers to call them, super- 

 families) and these into families, made up in turn of 

 one or more genera, each with one or more species- 



The beginner in Palaeontology is often troubled by the 

 importance attached in classification to the internal 

 characters of fossils or other features which are often 

 invisible in ordinary specimens, and is puzzled to under- 

 stand how identification is possible in such cases. It 

 may be pointed out that if one single specimen shows all 

 the characters that are necessary to determine its 

 systematic position, many others can be recognized as 



* Professor Schuchert, of Yale University, dissents from this 

 view, regarding articulation as a feature that has been evolved 

 independently in the orders Protremata and Telotremata. 



