258 EMBRYOLOGY OF TEREBRATULTXA. 



median line anterior. Thus the embryo possesses from twenty to twenty-five setae spring- 

 ing from the anterior margin. The larger and lateral bundles of setaj are the first to make 

 their appearance. They are nearly as long as the entire length of the emljryo, and are deli- 

 cately barbed and deciduous. Upon pressure of the embryo they readily separate from 

 their base of attachment. The dorsal valve is flattened, while the ventral valve is rounded, 

 as before remarked. The first evidence of hardened plates appears in the later stages of 

 these forms, for upon crushing the embryo in the hve cage a number of angular fractures 

 appear. (Plate ii.. Figs. 89 and 91.) 



Fig. 85, Plate ii., presents a view of an advanced stage, in which the relative position 

 and character of the deciduous setae are shown. 



Fig. 81, just above, represents the same embryo as it appeared when slightly crushed. 

 The manner in which the contents burst out from the shell still held by the membrane, as 

 in a sac, the fracture of the shell and the detached setae, are all instructive. Besides the 

 minute granules wliich make up the substance of the embryo, larger cells, to the number 

 of twenty-five or more, are seen scattered through the mass. In a shghtly advanced 

 stage. Fig. 90, Plate ii., the mouth shows on each side a rounded papilla, the first 

 appearance of the cirri. The mouth is prominent, and flush with the anterior borders of 

 the pallial membrane ; this is also shown in Fig. 82. 



Fig. 91 represents the appearance of Fig. 90 crushed. Here the incipient cirri show as 

 two prominences, one upon each side of the mouth. The shell fracture is also more appar- 

 ent. As the embryo advances in growth the outline changes, having at first a ti-ansversely 

 semicircular outline with the posterior margin straight, as in Leptaena. It gradually elongates, 

 and commences to widen its anterior margin, as in Fig. 93 ; it becomes well rounded as in 

 Fig. 88, and at this time is seen a difference in the length of the dorsal and ventral plate, 

 and for the first time appear concentric lines of growth. The deciduous seta?, at this stage, 

 separate at the slightest handUng of the embryo, and as in every case the figures on the 

 plates are exact fac-similes of my original drawings, the setae are not shown in Fig. 88, 

 as they dropped off in removing the specimen to the live box. A stage of the same age, 

 or slightly later, is shown in Fig. 94, and this stage unmediately precedes the formation 

 of the scaled structure of the shell. Its outline is quite unlil^e any adult Brachiopod 

 Ivnown. It is longer than broad. The posterior margin is quite broad and rounded. The 

 anterior margin is twice the breadth of the hinder margin, semicircular, and abruptly 

 rounded where it meets the lateral margins, which are incurved. The larger bundles of 

 seta3 project at the point where the anterior curved margin joins the lateral margin. (See 

 Fig. 94.) A distinct triangular area is seen within, which represents the head, below which 

 the digestive tract is faintly indicated. 



Sixth Stage. The embryo has now discarded its deciduous setae, and for the first time 

 the permanent setfe make their appearance, as well as the peculiar scaled structure of 

 the calcareous shell, and with these new features that of the peculiar tubules perforating 

 the shell must be added. 



Fig. 96 represents a somewhat more advanced stage than Fig. 94. The form is now 

 bluntly oval, its posterior margin prominent and bluntly roiuided. In general contour it 

 recalls Siphonotreta, placed in the family Discinida? by Davidson, a genus not occui'ring 

 above the Silurian. Seven prominent set£e longer than the shell now spring and diverge 

 from the anterior margin. These are not barljcd. The shell now plamly shows its scaled 



