42 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



of the Cambrian shells studied, nor could we find any in Walcott's sections, although they are 

 reported in Billingsella, Huenella, and Syntrofhia. All of the billingsellids and other Cambrian 

 articulates that we studied have the essential structure of Orthis and Clitambonites. There is there- 

 fore no reason to suspect that the shell structure in the oldest genera is different, and our evidence 

 favors the view of a uniform shell nature and growth of the orthids since early Cambrian time. 

 Furthermore, we find no punctate (endopunctate) shells in the Cambrian and Ozarkian, and this type 

 of shell is first seen in the dalmanellids (of the Chazy), which probably had their origin in early 

 Ordovician time (Canadian). 



Since brachiopods were first studied, investigators have noticed minute perforations in the inner 

 and outer surfaces of the test. The present observations show that these perforations are of two 

 main sorts and that they may be grouped into two divisions, as follows: 



ExopuNCTiE. — Pores of this type are readily visible on the unabraded exterior, i. e., in the outer 

 lamellar layer. They do not, however, penetrate far into the interior of the shell substance and 

 never pass completely through the fibrous test. Such pores are especially well exhibited by Pauror- 

 this and have frequently been mistaken for true punctas (endopunctse). They are also visible in 

 Valcourea, Doleroides, Hebertella, Rhipidofnella, and many other genera. In occurrence the 

 exopuncta; are sporadic within a genus and they are not confined to shells of the "impunctate" type. 

 In Valcourea, Doleroides, and some other genera, the exopunctse appear as perforations in tiny 

 granules on the ribs. 



Endopunct^. — These are pores that are found only in the fibrous layer of the test, extending 

 from the inside but never appearing on the surface when the outer lamellar layer is present. On the 

 inside they are common to the whole surface of the shell, but when the shell is costellate the pores 

 are concentrated into rows toward the exterior. 



VALUE OF PUNCTATION IN TAXONOMY 



Some European authors regard the endopunctje as important in the establishment of species 

 among living terebratulids and other punctate shells. Since the size and arrangement of the pores 

 is quite uniform, when the same portions of different shells are compared, a difference in pattern is 

 believed to be of specific value. The idea has not, however, found application in this country. 



Among the orthids, punctation is of great generic value, since certain types of internal structure 

 may be correlated with the endopunctas. The oldest known endopunctate shell occurs in the Middle 

 Ordovician (Chazy) but from the internal structure of some Lower Ordovician (Beekmantown) 

 genera, this type of punctate shell must occur at least that early. It appears evident at this time 

 that the punctate forms arose out of impunctate ones, but whether the punctate stock, once arisen, 

 remained persistent, or whether punctation was invented many times, has yet to be determined j it 

 would appear that both views are tenable. 



OLD-AGE CHARACTERS 



An important feature of the inside of a brachiopod shell is the internal thickening, which is a 

 consequence of age, since the mantle continues to secrete shell matter throughout life. This addition 

 of extra testaceous substance may so alter the internal features, muscle fields, and cardinalia in old 

 individuals as to suggest other genera than the one actually under observation. In the ventral valve 

 the chief alterations are in the obsolescence of the dental plates and the development of a pseudo- 

 spondylium. Hall and Clarke record the lack of dental plates as one of the generic features of 

 Orthorhynchula, but in young shells of O. linneyi and in O. ottawaensis (Bills.) there are prominent 

 dental lamellae. The development and importance of the pseudospondylium and spondyloid are 

 discussed under Glossorthis, Linoforella, Porambonites, and other genera having this feature. 



In the dorsal valve, adventitious tissue is commonly utilized for the support of the brachio- 

 phores, as is well exhibited in the Clitambonitidas (see figures of Estlandia). We have laid chief 

 stress in classification, however, upon the primary or youthful structures, and these have guided in 

 making our family groupings. The different modes of deposition of extra shell matter are useful in 

 some instances in showing relationships, but are more often deceptive. 



