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. 
* Whitfield, Ball. Americaa Maseum of Nat. History, vol. i, No. 8, p. 303, pi. xxiv, figs. 9-11. 1886. 
