MORPHOLOGY OF THE ORTHOID SHELL 31 



densed posterior area, which is but a sessile spondylium. [In some shells] the original contents of the pedicle-cavity 

 may be represented by enormous muscles whose scars extend almost to the anterior margin of the valve, as in 

 Hipfarionyx and Rhip'uiomella. 



The detail of the muscle-scars in brachiopods is usually difficult to determine, because of the 

 poor preservation of the inner surface, or because of the thinness of the shells. Furthermore, as the 

 animals grow larger, the muscles migrate forward, leaving more or less elongate tracks widening 

 toward the front. On the other hand, the tracks do not always afford the detail of the muscles 

 because the mantle later on covers up the posterior portions of the tracks as migration continues. It 

 is therefore not uncommon that a doubt may be entertained as to what the actual marks represent in 

 detail. To cover these various aspects of the muscle-scars we deem it best to adopt the following 

 terms from Buckman :"* 



1 . "Muscle-mark — any mark indicating muscular attachment. 



2. "Muscle-track — the course down the valve shown by successive muscular attachments. 



3. "Muscle-scar — more or less defined areas representing the ultimate muscular attachment." 



In general, when the preservation is favorable, four sets of muscle impressions may be distin- 

 guished in the ventral valve, as follows: (1) the principal or anterior diductors, (2) the adductors, 

 (3) the adjustors, (4) the accessory or posterior diductors. Of these four sets, the first three are 

 usually visible, but the accessory diductor scars are rarely to be seen, and in some instances the 

 adjustors are not determinable. 



( 1 ) Anterior diductor muscles. — ^The diductor impressions are always the largest scars in 

 the ventral valve. In the early impunctate genera, their tracks and scars are usually rather narrow 

 and straight, and never enclose the adductor impressions at the front. In later genera, such as 

 Dinorthis s. s.y DinorMs (Pl^siomys), and Rhipdomella, the diductor scars enclose the adductors. 



(2) Adductor muscles. — When the actual adductor scars are visible they are usually hemi- 

 elliptical in outline and separated by a low ridge which may be single or double. In some genera the 

 adductor impressions can not be determined, but their position is marked by a linear track. In a few 

 genera, such as Platystrofhia, Mcewanella, and Productorthis, there is no trace whatever of either 

 the scars of attachment or the tracks. In other genera (Schizophoria, Orthotichia, and the Entele- 

 tinae), the adductor muscles were borne on a high vertical septum, consequently their marks are not 

 recorded on the shell. 



(3) Adjustor or pedicle muscles. — These muscles are not always clearly defined on the 

 valves and have not been determined in all genera of the Orthoidea. The position of these mus- 

 cles, however, is always outside of, and posterior to, the diductors at the base of the dental lamella, 

 or they are more or less elevated on the side of these plates. As would be expected from their posi- 

 tion, these muscle impressions are usually elongate, and rarely present the semi-flabellate outlme 

 commonly seen in the diductor scars. The adjustor scars are the most variable of the muscle mark- 

 ings and in this work little reliance has been placed on them in the establishment of genera. Where 

 good material is available, however, they may be of some use in the making of speaes. 



In their great work on brachiopod genera. Hall and Clarke"' have applied two terms to the set 

 of muscles we are calling adjustors. In the text of the discussion of DinorMs they refer to these 

 muscles as "adjustors," but in the legend to Plate V, where this genus is figured, they call them 

 "posterior diductor muscles." We prefer to use the term adjustor muscles because these scars have 

 precisely the same position in reference to the diductors as in Recent genera. Furthermore, the pos- 

 terior diductor scars in living forms are located just posterior to the adductor impressions. Similarly 

 situated scars can occasionally be seen in Paleozoic shells and their presence lends support to our 

 usage of the term adjustor. It must be admitted that similarity of position does not prove homology 

 of the muscle-scars in question, but in the absence of more definite knowledge of the soft parts of 

 these Paleozoic shells, we can only compare the structures in the older genera with those in the 

 Recent forms. 



"Brach. Namyau beds, 1917 ( 191 8), p. 90. 

 »« Pal. N. Y., vol. 8, pt. 1, p. 195. 



