ANATOMY OF THE TEREBRATULA. 15 



If the dorsal valve and corresponding lobe of the mantle be removed, as in pi. 8, fig. iii, 

 and the 'musculi retractores superiores,' t, be gently divaricated, or the mesial fasciculi 

 carefully removed, as in fig. ii, the delicate membrane of the venous sinuses, ib. fig. ii, 7, 

 and fig. i, 6, communicating with and closing the basal apertures of the auricles, i, is 

 immediately exposed; and is so transparent as to permit the plicated structure of those 

 cavities to be clearly seen. 



If the viscera be exposed by a side view, as in pi. 1, fig. 1, the heart of the side 

 exposed will be seen behind the beginning of the intestine, as at i, fig. 1. 



The ventricle, 2, pi. 3, fig. 1, in each heart, is a smooth oval feebly-muscular cavity, which 

 transmits the blood by two arteries, — the largest to the two halves of the mantle-lobes nearest 

 the ventricle, the smallest to the viscera and muscles. In the mantle the arteries terminate 

 at the periphery in the circum-pallial vein or sinus, 4, pi. 2, fig. ii, from which the branches, 

 ib. 5, 5, of the large pallial sinuses commence. These sinuses, although the cylindrical is 

 exchanged for a flattened depressed figure, retain enough of the more normal character of 

 the veins to be recognised distinctly as such. There are two of these branched sinuses, 5, 

 in the dorsal lobe, or that wdiich lines the imperforate valve, and four, g, in the ventral lobe, 

 or that which lines the perforated valve. Their longitudinal course and relative positions so 

 closely correspond with those which I have described and figured in Terebratula chilensis} 

 as to preclude the necessity of repeating either description or figures in this place. The 

 two sinuses in each half of the ventral lobe unite into one, and this trunk unites with the 

 base of the single sinus of the corresponding half of the opposite mantle lobe ; the 

 common sinus, so formed, at the back part of the visceral chamber receiving the venous 

 blood from other sinuses there that fill, line, and seem to form, the visceral or peritoneal 

 cavity. The common sinus so constituted, on each side of the intestine, terminates by 

 being continued into the margin of what seems to be the widely-open auricle, as seen 

 through the transparent parietes of the sinus, 1, pi. 3, fig. 1. The auricular cavity would, 

 however, be more correctly described as a closed one, consisting at the half next the ventricle 

 of a beautifully plicated muscular coat, in addition to the membranous one, but at the other 

 half, next the venous sinuses, of venous membrane only : the latter might be termed the 

 auricular sinus, the former the auricle proper. The proper auricle, then, ib. 1, 1, presents 

 the form of an oblong depressed cone, attached by its apex to the ventricle, the apex being 

 penetrated by the auriculo-ventricular aperture, and adherent by its base to the auricular 

 sinus, which might be said to conduct into the common peritoneal cavity, since the 

 delicate tunics of the visceral sinuses appear to take the place of a peritoneum. The 

 muscular walls of the auricles are thin and delicate, but exhibit two layers of fibres, 

 the exterior ones being disposed transversely or circularly, the interior ones longitudinally, 

 the latter being the most delicate and radiating from round the perforated apex to the 

 plicated circumference of the auricle. These muscular walls are beautifully disposed in 



^ Tom. cit., pp. 147, 148, pi. xxii, figs. 5 and 6. 



