BRACHIOPODA. 217 



Triplesia lateralis* is a transversely elongate, biconvex shell, with a straight 

 hinge-line whose length nearly equals the greatest diameter of the valves, and 

 each valve is medially divided by an open delthyrium. The external surface is 

 smooth, with fine concentric lines visible only about the margins ; the inner 

 shell-layers show a strongly fibrous radiating structure without punctation. 

 The pedicle-valve bears a more or less clearly developed median sinus and 

 the brachial valve a broad, indistinct fold. 



On the interior the teeth are very small, lying at the extremities of the 

 delthyrial margins and supported by dental plates which converge and unite 

 before reaching the bottom of the valve. Thus is formed a deep but short 

 spondylium, which is supported, near its apical portion, by a median septum, 

 but is free for fully one-half its length. 



In the brachial valve there are also two convergent plates bounding the 

 deltidial cavity, larger and stronger than those of the opposite valve. These 

 plates may rest upon the bottom of the valve, and probably always do so 

 toward the posterior extremity, but anteriorly they become free, forming a 

 spondylium which is supported by a median septum extending beyond the 

 anterior edge of the plate. Thus these two valves, which are very similar in 

 exterior, the pedicle-valve being only slightly the more convex and with a low 

 median sinus, are also closely alike on the interior, each being furnished with a 

 spondylium. 



Adhering to this species, as typical of a peculiar generic structure, there 

 seems no reason to doubt that Billings' species Stricklandinia ? Arachne and 

 S. Arethusa should be associated with it. They are externally of the same 

 character except that the surface of the former bears obscure radiating plica- 

 tions. On the interior the septum supporting the spondylium is longer and 

 projects anteriorly, and the description of S. ? Arachne states that in the brachial 

 valve there is no median septum. While we have not had the opportunity of 

 examining the originals of these species, it may be observed that in Syntrophia 

 lateralis this septum is .so delicate as to be detected with difficulty in prepara- 

 tions of the interior, but transverse sections of the valves do not fail to reveal it. 



• Whitpibld, Bull. American Museum of Nat. History, vol. i, No. 8, p. 303, pi. xxiv, figs. 9-11. 1886. 



