3U 



CAMBEIAN BRACHIOPODA. 



PEDICLE TUBE AND APERTURE. 



ATBEMATA. 



When the pedicle rests in furrows on the areas of the valves, as in Oholus (Pis. VII and 

 XIV), it pertains to both valves, but mth the deposition of a narrow band of shell about it on 

 the ventral valve it strictly pertains to this valve alone, although it may still rest against 

 the area of the dorsal valve before passing through the semitube of the ventral valve. The 

 transition from the deep but open pedicle furrow of the ventral valve of Oholus to the inclosed 

 sheath or tube of Ololella is found in BiceUomus politus. In this species there has been a deposit 

 of shelly matter that rises up from the sides of the pedicle furrow on the ventral valve and 

 arches slightly inward, almost forming a tube in places; what remains of this semitube is shown 

 by figures 16A and ICB. 



NEOTBEMATA. 



The earliest and simplest stage of development of the pedicle tube when it completely 

 surrounded the pedicle is found in Oholella of the Lower Cambrian (Pis. LIV and LV). In. this 



genus the deep pedicle furrow of the ventral valve 

 of Dicellomus has been covered over just at the 

 apex of the cardinal ai'ea so as to form a simple 

 ring or tube about the pedicle. In the next stage, 

 as shown by Botsfo'rdia (Pis. LVII and LIX), the 

 pedicle has advanced its opening to the front side 

 of the apex and the tube between it and the furrow 

 in the area is a little stronger and a trifle larger. 

 In Linnarssonella (Pis. LXXVIII and 

 LXXIX) the opening of the pedicle tube is close 

 to the posterior margin and the Obolella-like struc- 

 ture in both valves is very striking. This is best 

 seen by comparing the casts of the interior of the 

 ventral valves of Linnarssonella (PL LXXIX, 

 figs. 3c-g) with those of Oholella atlantica (PI. LV, 

 figs. If-h). In both species the cast of the pedicle 

 tube is in the same position in relation to other 

 parts, and the external opening of the pedicle tube 

 is at the apex above a low false cardinal area. 

 The depressed forms of Acrotliele, with the apex near the posterior margin, appear to have 

 been descendant from the Oholella stock but to have reached a stage of development more 

 advanced than any known form of the Cambrian genus Obolelh'. Acrothele hellula (PI. LVIII, 

 figs. 5, 5a-h) of the Mddle Cambrian appears to be one of the oldest forms and to have retained 

 in its ventral valve characteristics of its obolelloid ancestors. In the ventral valve the pedicle 

 tube penetrates the thickened apex of the shell and there is considerable space between the 

 pedicle aperture and the posterior margin; the tube and aperture are essentially the same in all 

 the species of Acrothele, difi'ering only in details due to a rather narrow range of position and the 

 varying thiclmess of the shell. The genus ScMzopliolis (PI. LXXXI, figs. 1, la-c) has a more 

 nearly vertical posterior slope and distinctly marked false area, in this respect resembling 

 the large group of species referred to Acrotreta. The pedicle tube and aperture of the latter 

 are of the same type as in Acrothele, differing only in form and position within a narrow range. 

 Acrothyra also shows little variations. Discinolepis (PI. LXXXI, figs. 3 and 3a) has the pedicle 

 opening more advanced and appears to be a form intermediate between Acrothele and Acrotreta. 



Figure 16. — Dicelhmus politus (Hall). A, Interior of ventral valve 

 (f.sp, pseudospondyliura or pedicle plate; i, transmedian scars; j, 

 anterolateral scars; v, visceral cavity; vs, main vascular sinuses). 

 B, Posterior portion of the interior of a ventral valve, showing 

 a strongly developed area (fsp, pseudospondylium or pedicle 

 plate; a, inside section of area; f, flexure line; a', outside section 

 of area). 



The specimens represented by figures IGA and 16B are from 

 Locality 79s, Upper Cambrian "St. Croix sandstone" at Hudson, 

 Wisconsin (TT. S. Nat. Mus. Cat. Nos. 51917aand 61917b, respec- 

 tively). The same figures are reproduced on page 570. 



