INTRODUCTION 3 



send them forth as a contribution to the evolutionary history of the brachiopods — a study the 

 importance of which was well seen by Charles Darwin when he unavailingly appealed to the lead- 

 ing specialist of his time, Thomas Davidson, to take up the brachiopods from the standpoint of 

 descent through evolution. 



Location of Specimens. — As indicated above, this memoir is based mainly on material in the 

 Schuchert Brachiopod Collection in the Peabody Museum of Yale University. Specimens from 

 this collection have the letter S preceding the catalogue number (e. g., S 12). Specimens from other 

 collections in the Peabody Museum have the letters Y.P.M. following the catalogue number 

 (e. g., 120 Y.P.M.). 



Technique. — In any critical revision of brachiopod genera it is paramount to know every detail 

 of the shells with which one is dealing. It is not difficult to find brachiopods in the field showing 

 the exterior, as free specimens or as molds in the rock; it is far more difficult to find free valves 

 with the interiors cleaned by nature, or as sharp molds. In the Schuchert Collection by far the 

 majority of the specimens are free individuals showing the exterior in good detail. This is a con- 

 sequence of the collection's being a composite one, built up of choice material brought together by 

 many skilled collectors. To be sure, there are great numbers of interiors in the collection, but not 

 of every genus. Accordingly, it became necessary to prepare interiors for a large number of the 

 genera that we studied. We were able, by various methods, to obtain the essential details of the 

 internal anatomy of every genus with which we worked. There is really no reason why, given 

 plenty of material, the interior of every genus of brachiopods should not be known. 



The following methods were used in securing the internal anatomy of the brachiopod shell: 

 (1) etching, (2) cleaning with needles, (3) burning, and (4) serial sectioning (Zugmayer process). 



( 1 ) When specimens of dissociated valves are filled with shale or clay, the internal surface 

 may be obtained by placing on the specimen lumps of potash and allowing the chemical to deliquesce. 

 The potash then attacks the shale and loosens it. This method is not always satisfactory, however, 

 because the potash usually attacks the substance of the shell, producing a white efflorescence which can 

 be got rid of only by the use of dilute acid. This means the loss of some details of the surface, and 

 is undesirable when other methods can be pursued. 



When shells are silicified they can as a rule be freed from the limestone by dissolving the 

 matrix in dilute hydrochloric acid. Care must be taken, however, to use exceedingly dilute acid, 

 otherwise the shell itself may be attacked or be disrupted by violent ebullition of gas. It is not usual 

 to find shells properly silicified, but when one does, they show the internal characters exceedingly 



well 



(2) More laborious, but more satisfactory by far, than the use of potash is the careful manipu- 

 lation of a needle accompanied by a dentists' drill. With this combination the interior of most 

 brachiopods can be obtained. We found it most satisfactory to cement the specimen in plaster of 

 paris and then excavate the interior with a needle, carrying on all the manipulations under a binoc- 

 ular microscope. This method requires time and patience, but when one has prepared successfully 

 the interior of a rare shell, he is amply repaid for the expenditure of one-half to a day's tirne. In 

 some instances we took specimens in which both valves were in contact and secured the interior 

 of one or the other valve, depending on which we wanted. If the ventral interior was desired, the 

 dorsal valve was ground off, the remaining one cemented in plaster and excavated by the use of 

 a needle. The majority of the interiors figured in this memoir were prepared by the use of a needle. 



(3) John M. Clarke and S. S. Buckman were the pioneers in the use of the method of burn- 

 ing or calcining the shells of brachiopods. This method was used by the latter to good advantage 

 in his splendid treatise on the Burma brachiopods. Buckman heated the shells to redness and 

 dropped them into water, the shell spalling off in the process. We found, however, that the drop- 

 ping of the shells into water was disastrous to the specimen in certain types of matrices. It is more 

 satisfactory to heat the shells in air to redness, and, after cooling, to scrape the shell off with a sharp 

 needle The process of heating softens the shell and allows it to be removed rather easily. Natural 

 casts of interiors made by this method, and internal molds in general, have their limitations; they 

 are excellent for details of the muscle-scars, but it is usually difficult to get sharp impressions of the 

 cardinalia, which are best seen in free valves. ^-«.,- 



'^•LICR/ 



\^ 



