28 GENERA OF THE SUBORDERS ORTHOIDEA AND PENTAMEROIDEA 



Kozlowski^* has recently made a detailed study of the spondylium in the Pentameracea and has 

 reached some striking conclusions. He has shown that there are two types of spondylia in the Pen- 

 tameracea as constituted by Schuchert in 1913. One of these types, in which the spondylium is sup- 

 ported by a simple septum, he calls the "spondylium simplex," and thinks it originated from a 

 pseudospondylium by lateral crowding of the ovarian bodies and consequent resorption of the deposit 

 forming the base of the pseudospondylium. The Clitambonacea are characterized by this type of 

 spondylium and Kozlowski finds this superfamily to be more closely related to the orthids than to 

 Pentamerus. In his conclusion we concur, having arrived at the same opinion independently but 

 using different criteria as well, namely, the pallial and ovarian impressions and the cardinalia. 



Characteristic of the second group — the Pentameracea — is the "spondylium duplex." In this 

 type the spondylium is elevated on a septum which is clearly composed of two parts thought to have 

 been produced by the squeezing of the two dental plates together due to lateral pressure from the 

 ovarian bodies. This type of spondylium is common to the pentamerids, but the cardinalia are much 

 more characteristic than is a spondylium duplex which is known in many other genera. 



Present views and definitions. — We may add that the dental lamellse and the floor of the 

 valve between them are in many genera bound together by a chamber called the delthyrial cavity, and 

 in most of the orthids the floor of this space is the seat of attachment of the various muscles. In 

 other orthid genera the dental plates converge and unite with each other medially on the floor of 

 the valve, or are supported by, and fused with, a median ridge before attaining the shell floor, 

 forming an elevated spoon-shaped structure. It is to the latter type of muscle platform that the term 

 spondylium is now restricted. In rare instances among the oldest brachiopods {Protorthis) the dental 

 lamellae unite to form a freely suspended spoon without the intervention of a median septum. These 

 various types of muscle platforms may be briefly defined as follows: 



1 . Discrete spondylium. — This, Kozlowski's spondylium discretum, designates the condition 

 in which the dental lamellae converge but slightly and extend directly to the floor of the valve, but 

 never become united. This type of muscle cavity occurs in PoramboniteSy Huenella, and a host of 

 other orthid shells. It is not a spondylium, but the structure does suggest how the genuine spondylia 

 arose, namely, by the complete convergence and union of the dental plates above the floor of the 

 valve, and their support by a median septum standing on the floor of the shell. 



2. Pseudospondylium. — This term was introduced by Walcott^^ for "the rudimentary spon- 

 dylia attached directly to the inner surface of the valve, as in BilUngsella." In all the species listed 

 under the genus BilUngsella in the present work the dental lamellae do not converge but extend 

 vertically to the floor of the valve and conform in all essentials to the type of muscle platform 

 called by Kozlowski the spondylium discretum. In some species, however, the anterior portion of 

 the muscular area is elevated on a callosity. This shell thickening within the delthyrial cavity and 

 its connection with the dental lamellae make it appear that these plates are united but in reality they 

 are discrete, resting on the floor of the valve. It is to this type of muscle platform that we propose 

 to limit the term pseudospondylium. 



Pseudospondylia are excellently exhibited in Glossorthis (see pi. 4, figs. 9, 12), Linoporella 

 (see pi. 18, figs. 13, 14, 18), and Finkelnburgia (see pi. 13, figs. 6, 11). In these genera the callus 

 beneath the muscle attachments is extended from the floor to the inside face of the dental lamellas. 

 Anteriorly the callus is thick and in Finkelnburgia is extended forward as a low ridge. Specimens of 

 Linoforella kindly loaned by Dr. A. H. Westergaard of Stockholm show a complete series from 

 the young stage with discrete dental lamellae and unthickened muscle attachments to a condition in 

 which a typical pseudospondylium with a median ridge has been developed. This series is illustrated 

 on plate 18. 



Pseudospondylia are naturally more developed in old or late mature shells, as young individuals 

 do not deposit much callus. Since, therefore, the pseudospondylium is a feature of mature shells 

 only, it is obvious that it has a minor value in the taxonomy of the Brachiopoda. 



"Pal. Polonica, 1919, pp. 122-125. 

 "Camb. Brach., 1912, p. 295. 



