PART I CHAZYAN AND RELATED BRACHIOPODS—COOPER 717 
TITANAMBONITES Cooper, new genus 
(Greek titan, large; ambon, umbo) 
Shells usually large, generally more than an inch wide; concavo-convex and 
resembling Sowerbyites in exterior details: Costellae fine, distant, crossed by 
prominent concentric fila. Shell substance pseudopunctate. 
Pedicle interior with large, flat, oblique teeth having a deep crural fossette ; 
dental plates absent ; muscle field large, resembling Sowerbyites with long diduc- 
tor scars extending to the middle or beyond and lying at the base of large cal- 
losities ; adjustor scars long and slender, located posterolaterally to the diductors ; 
adductors constituting a small somewhat rectangular patch just anterior to the 
posterior junction of the diductors; ovarian impressions small. Pallial impres- 
sions prominent, vascula media extending anteromedially from anterior end of 
diductor scar and occupying the callosity at the end of the diductor scar; minor 
distributaries occupying the inside of the trail. Pseudodeltidium a long, narrow 
arch. 
Brachial valve with low median septum not prominent anterior to the cardi- 
nalia but rising to a crest anteriorly and partitioning the shell cavity where the 
trail is geniculated from the body. Brachiophores small, short, divergent; car- 
dinal process a boss excavated on the side facing the anterior but with a trisep- 
tate myophore, the median ridge being the highest; chilidium narrowly convex, 
arched over proximal end of cardinal process; cardinal process supported by 
shell substance deposited between the brachial process and the side of the cardi- 
nal process. Adductor field small, the posterior adductors the larger; pallial 
trunks strongly impressed, a large trunk extending anteriorly on each side of 
the median ridge and another major trunk extending anterolaterally from place 
of junction of anterior and posterior adductor pairs. Ovarian impressions mod- 
erately well developed. 
Genotype.—Titanambonites medius Cooper, new species. 
Discussion—The members of this genus are fairly easy to recognize because 
of their general resemblance to Sowerbyites. They are, however, usually much 
larger than any known members of that genus and the brachial valves are very 
different. Titanambonites has an interesting ornamentation which is unlike the 
usual plectambonitids in not having the fine matrix of extremely small costellae. 
In the genus under consideration the costellae are moderately distant and appear 
in several generations. They are crossed by fine but prominent concentric fila 
which cover the whole shell. In this respect Titanambonites resembles the 
Esthonian genera Leptestia and Ingria. In this respect, too, it is unlike Sower- 
byites. 
The pedicle interior contains a number of interesting anatomical features. Den- 
tal plates are absent, but the spaces between the palintrope and the inner shell 
surface are the sites of thick shell deposit, so thick in some specimens that the 
shell material protrudes anterior to the margin of the palintrope to form a slightly 
oblique ridge. The muscle field with its prominent callosities anterior to the di- 
ductor scars is very suggestive of Sowerbyites. The callosities are usually very 
