PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 517 
more primitive (or possibly more specialized) members of the family in not 
being provided with complete deltidial covers. Apomatella has an open del- 
thyrium, and Afelelasma is distinguished by the presence of lateral plates that 
grow obliquely toward the center of the delthyrium. 
Absence of modifying plates over the delthyrium among the Clitambonitacea 
may be a primitive character or it may be the reverse. Not enough is yet known 
of the development of these forms to be sure which is the correct view. Among 
the earliest of the Orthidae a deltidial cover is common, and its absence in the 
later members suggests specialization. In the instance of Apomatella and Atele- 
lasma, which are stratigraphically early members of the family, absence of the 
covers may indicate primitive rather than specialized characters. 
ATELELASMA Cooper, new genus 
(Greek ateleo, imperfect; elasma, plate) 
Planoconvex to unequally biconvex; hinge wide; interarea of pedicle valve 
long and generally flat; delthyrium modified by lateral plates only. Anterior 
commissure rectimarginate to gently sulcate; surface multicostellate, costellae 
often swollen and hollow. Impunctate. 
Pedicle interior with broad spondylium simplex supported by a short septum. 
Vascula media usually strong and located on each side of the median septum. 
Brachial valve interior with brachiophores in the form of thin, flat blades sup- 
ported by swelling of the thick notothyrial platform. Cardinal process a simple 
septum. Chilidium prominent. Median ridge thick, elevated and extending to 
about the valve middle. 
Genotype.—Atelelasma perfectum Cooper, new species. 
Discussion.—This genus is characterized by its Vellamo-like form and usually 
open delthyrium which is only modified by erect lateral plates. In this respect 
it differs from all other members of the Clitambonitidae, which have a strong 
pseudodeltidium with large subapical foramen. It differs from Apomatella in 
having a higher organization, usually apsacline interarea, and a well-developed 
chilidium which is absent from the European genus. Apomatella has rudimentary 
lateral plates, but that genus is a more primitive one than Atelelasma. 
Atelelasma differs from all members of the Estlandiidae in having an impunc- 
tate shell. 
Schuchert and Cooper (1932, pl. 7, figs. 5 and 11) figure a specimen of Atele- 
lasma multicostum (Hudson) having an imperforate, short pseudospondylium 
at the apex. This is the only specimen of the kind yet seen of this genus and the 
only occurrence of such a plate known in it. None of the silicified specimens pre- 
pared from the Arline formation showed any trace of such a plate. This pedicle 
plate is low and lies just under the edges of the palintrope. Other specimens of 
A. multicostatum are provided with lateral plates like those of the type species. 
This plate may be a pedicle attachment lost from the other known specimens as 
may be the case with certain spiriferoids, or it may be an atavistic occurrence, 
the importance of which is not yet understood. 
