MORPHOLOGY OF THE ORTHOID SHELL 



33 



In the Orthidas and Clitambonitidas the two main trunks are clearly visible but instead of being 

 widely divergent as in the Billingsellidas they extend forward near the mid-line of the shell, being 

 separated by a narrow septum only (t. figs. 3, 4; pi. 2, fig. 18; pi. 6, figs. 18, 26; pi. 8, fig. 10). 

 At the front they may or may not diverge. In the higher families of the Orthoidea such as the 

 Dinorthidse and Plectorthida: the pallial markings are more complicated but of the same fundamental 

 pattern. In Plcesiomys (see t. fig. 5; pi. 10, fig. 26) the main trunks are short and rather narrow, 

 diverging from the antero-lateral extremities of the diductor scars. The main trunks branch almost 

 immediately after their inception, sending one trunk posteriorly and another antero-medially. Each 

 of these subsidiary trunks gives rise to minor branches toward the front and sides of the shell. In 

 Mhnella (see t. fig. 6; pi. 12, fig. 20) the general arrangement of the trunks is on the plan of 

 D'morthis, but details of the branching are different. Orthostrophia (t. fig. 7) has the two diverg- 

 ent trunks of the Dinorthidx which branch into subsidiary trunks, but in addition has one or two 

 minor trunks extending forward from the adductor tracks. These may actually be branches from the 

 main trunks in front of the diductor impressions, but no lines of contact were observed. 



In the Dalmanellidaj the two main trunks are visible in many of the genera, extending forward 

 from the diductor impressions. The branching of the main trunks into subsidiary rami is more or less 

 complicated (t. fig. 8; pi. 17, fig. 22). 



^^.<«'< 



Figs. 6-8. — Ventral pallial markings. 6, Mimella mdonica (Willard). 7, Orthostrofhia aff. O. Urofhomenoides 

 (Hall), in which they are of the same type as those of Dinorthis and Mimella. 8, Dalmanella ignota Sardeson: Pc, pedicle 

 callist; da'», accessory diductor scars; /la!;, adjustor scars; i4</, adductor scars; Z)», diductor scars. 



Genital or ovarian markings.— In living brachiopods the genital glands (brachiopods are 

 dioecious) are, according to Thomson,^ 



rather thick, convoluted bands . . . varying somewhat in size . . . There are four glands in each animal, two on 

 the ventral and two on the dorsal side . . . The glands in some genera are bound down to the shell by muscular 

 ties, which give the shell a pitted appearance, the so-called ovarian impressions. 



These markings are usually dissimilar on either valve, and in some Orthacea are not observable 



at all. • L u 1 1 



Genital or ovarian markings consist usually of more or less reniform areas m the umbo-lateral 

 portions of the valve bounded by the main trunks or the posterolateral branches of the pallial sinuses. 

 Within the reniform areas are elevated ridges radiating from the umbonal cavities. These probably 

 represent attachments of the ovarian bodies. Reniform ovarian areas are most promment in the 

 Orthidse and Clitambonitidx, but in many genera these markings are not represented by reniform 

 areas but by wavy ridges or by pustules that are aggregated in the umbo-lateral spaces. 



"Brach. Morph., 1927, p. 13. 



