MORPHOLOGY OF THE ORTHOID SHELL 25 



directed toward the delthyrium. The teeth are considered to extend vcntrally as far as the ridge 

 bounding the crural fossette (see below) on the inner face of the tooth. The progressive forward 

 growth of the tooth produces a thickening under the palintropc along the dclthyrial margin, and 

 likewise a triangular area on each side of that margin. This small area is bounded on the inside by 

 the delthyrial margin and on the outside by a more or less well defined suture line connecting the beak 

 with a small socket in the outside, posterior face of the tooth. These narrow triangular spaces usually 

 can be easily differentiated from the remainder of the interarea by the difference in color. In punc- 

 tate shells the palintrope is punctate, but the dental area is impunctate. In a cross section of the 

 palintrope the triangular area is also three-sided and is supported by the dental lamells. 



The teeth of Rhipdomella vary from the usual type described above. They form the margin 

 of the umbonal cavity and rest on the floor of the valve with little or no supporting plates, and 

 project dorso-laterally as blunt points. 



Sockets. — There are sockets in both valves which receive articulating processes from the oppo- 

 site shell. In the ventral valve there are two kinds of sockets associated closely with the teeth: 

 (1) Accessory dental sockets in the postero-lateral face of the teeth (see t. fig. 2); and (2) a crural 

 fossette or crural socket" on the inner face of the teeth. 



1 . The accessory socket in the tooth of the ventral valve is located in the postero-lateral sur- 

 face of the tooth next to the hinge-line and on the outside margin. It articulates with a small 



SfS 



Y^r.. 2. — Diagram showing the ventral interarea of Vdcourea, with accessory sockets in the teeth {Acs). 



apophysis or tooth situated on the outside of the dental socket of the dorsal valve. This accessory 

 articulation is common throughout the Orthoidea and in strophomenids, spiriferids, etc. 



2. The crural fossette is a more or less deep groove, usually defined by an oblique ridge, located 

 in the inner face of the tooth just ventrad of the delthyrial margin. Not uncommonly the fossette 

 is not sunk deeply beneath the surface of the tooth but is made by a small subtriangular plane sur- 

 face defined by an oblique ridge, a type frequently seen in the family Orthidx. The function of the 

 crural fossette is to articulate with the carinate, or, in some genera, expanded postero-ventral edge or 

 face of the brachiophore. In Hesperorthis the fossette is small, but the oblique ridge defining it is 

 prominent. In the articulation of this shell each tooth fits into the socket which is located outside 

 the brachiophore; it extends into the socket a distance equal to the plane or sunken face of the 

 crural fossette, while the oblique ridge rests on the posterior or postero-ventral edge of the brachio- 

 phore. It follows then that the brachial apparatus of the orthid plays an important role in the artic- 

 ulation of the shell, a role no less important than that of the teeth and sockets. 



The crural fossette of Parmorthis (pi. 21, fig. 10) is developed to a remarkable degree. It is 

 an elongate, deep groove extending nearly to the floor of the valve and terminating in a deep socket. 

 The oblique groove receives the postero-ventral edge of the crural base, and the socket at the end 

 of the groove articulates with a slight expansion near its ventral extremity. 



Dental plates. — Beneath each tooth there are as a rule two more or less strong plates divid- 

 ing the space beneath the overhanging palintrope into three chambers, two lateral ones and a central 

 one the delthyrial cavity. These plates have been most commonly termed "dental lamellae" or 

 "dental plates." Thomas calls them "delthyrial supporting plates" and Fredericks speaks of them 

 as "lamellae apicales." We prefer to remain by the older terms "dental plates" or "dental lamella." 



"The term crural fossette ("fossette crurale") was used first by Kozlowski (Bib. Univ. Lib. Polon^, A, 1927, fasc. 

 17, p. 8, fig. 1). 



