MORPHOLOGY OF THE ORTHOID SHELL 21 



area the writers consider the length of said surface to be in the same direction as the length of the 

 valve. The width of the interarea is the width of the hinge-line. The length of the interarea is thus 

 seen to be perpendicular to the width or perpendicular to the hinge-line. Ordinarily the ventral 

 interarea has the greater length. 



An interarea may be plane or more or less markedly concave. It may be striated parallel to 

 its length or to its width or both, most interareas having striae of one kind or another. The hori- 

 zontal stris are commonly growth-lines; the vertical stria: may represent the growth tracks of acci- 

 dental irregularities that make, as it were, "teeth" along the hinge-line. More uncommonly the 

 interareas may be ornamented by fine elevated lines passing obliquely across the surface from the 

 beak, as in Polytcechia and Deltatreta (see pi. 6, fig. 14). 



Another important feature of the interareas is their inclination from the plane of the commis- 

 sure, which may cause important modifications of the muscle marks, the dental plates, and the 

 cardinalia. When studying the inclination of the interareas the ventral valve is placed by us below 

 the horizontal and the beaks (arbitrarily) on the observer's left. The plane of the commissure is 

 then horizontal and the dorsal valve is up. In this orientation the following positions may be 

 observed : 



Ventral valve: (a) anacline — ^between vertical and horizontal — ex. Orthis s. s.; (b) orthocline — 

 interarea horizontal — ex. Archceorthis ; (c) apsacline — between horizontal and vertical, 90°-180° 

 from erect vertical — ex. Hesperorthis; (d) catacUne — bent down 90" from the horizontal — ex. some 

 species of Clitambonites; (e) frocUne — interarea bent more than 180° from the vertical — ex. Din- 

 orthis {Retrorsirostra). In the dorsal valve the anacline, orthocline, and apsacline conditions are 

 recognized, and in addition, a hyfercline condition in which the interarea is rotated in an anterior 

 direction more than 90° (see t. fig. 1). 



This nomenclature has the advantage of enabling the observer to state precisely the average or 

 individual inclination of either interarea, or, if he desires to be more specific, the number of degrees 

 subtended by the interareas or by either interarea and the horizontal. For example, one may define 

 a certain interarea as 30° apsacline. 



Of interest in connection with the interareas is the presence of a narrow triangular space on 

 either side of the delthyrium or notothyrium. One side and the base of the triangle are formed by 

 the delthyrial margin and the hinge-line, respectively. The other side is defined by a line running 

 from a point outside of the tooth in the ventral valve, or the socket in the dorsal valve, to the beak. 

 This line represents a suture marking the progressive growth of the tooth and socket; it is the line 

 between new shell deposited on the tooth or in the socket against the old shell of the hinge-line 

 during the growth of the valve. In the Strophomenacea a similar triangular space has been called 

 the "secondary area," but in these shells, Derbyia, Orthotetes, etc., the outer or suture line is never 

 correlated with either the teeth or the sockets. It is very doubtful, then, if this better known and 

 much discussed secondary area of the Strophomenacea can be homologized with the similar-appearing 

 structure in the orthoids. 



DELTIDIUM 



Modifications of the delthyrium (deltidium). — ^The deltidial covering has for many 

 years received much attention from taxonomists, and the kind of covering is now thought to deter- 

 mine the order to which the different groups of brachiopod shells belong. The deltidium' is an arch 



^According to Hall and Clarke (Pal. N. Y., vol. 8, pt. I, p. 189, footnote), "The term deltidium was proposed by 

 von Buch for the triangular flate which, in many articulate genera, covers more or less completely the space between the 

 outer margins of the dental ridges. This plate he describes a composed of two pieces which may either completely sur- 

 round the foramen {deltidium amflectens), bound it on its lower side {deltidium sectans), or the parts may be separated 

 for their entire length by the foramen (deltidium discretum). These component parts of the deltidium take their origm 

 from the margins of the triangular cavity beneath the beak, but in some genera, particularly in Strofhomena, Sfirijer, and 

 their allies, there is still another form of shelly plate which grows from the apex downward, and to this the term peudo- 

 deltidium was applied by Bronn. Among recent writers there has been considerable laxity in the use of these terms and it 

 is very doubtful if they can be applied with precision." _ , , .,. 



In the present work we have preferred to follow Hall and Clarke, Beecher, and others in using the tenn deltidium, 

 applying it, however, to the cover composed of a single piece which restricts the delthyrium of the Protremata. 



