700 DEEP WATER MOLLUSES AND BBACHIOPODS— BALL. vol.xvii. 



II 



aiul sliow the marbled whitish and purple coloration of the interior. 

 The purple matter tinges fresh water of a j-ellowish ainber eolor. 



The anterior portion of the foot resembles that of Uuciroa, and it 

 shows a small byssal groove, from which an extremely slender byssal 

 thread or two proceeds, and was observed by Verrill. But behind the 

 byssal groove, on the median line of the visceral mass, is produced a 

 thin, compressed, fin-like bod^^, which I propose to name the opistho- 

 podium, and which in life may be iicarly flat v^ertically, but in alcoliol 

 is so contracted as to cockle the distal margin, giving the organ a 

 peculiar aspect, entirely unlike that of any pelecypod foot I have ever 

 examined, and strikingly like a fin. Something analogous was described f 

 by Owen in Fholadomya. The retractors of the foot are double on 

 each side for more than half their length. The attachment is behind 

 and somewhat separated from the anterior adductor in the case of the 

 protractors, while the retractor scar touches the upi)er angle of the 

 posterior adductor. 



On sei)arating the mantle lobes and examining theroof of theperipedal 

 cavity we observe a large visceral cone of oblong section, at the anterior 

 lower extremity of wliich is the functional foot, while behind the latter 

 is the fin like exi^ansion I have already referred to. Closely embracing 

 the base of the iiedo-visceral cone, and extending forward to the mantle 

 at the sides of the month, partly covered by the free edges of the palpi, 

 and backward to the siphonal septum, are the ctenidia. These are the 

 morphological equivalents of a single gill stem on each side, with lateral 

 exi»ansions; on the one hand closely appressed to the side of the foot, 

 on the other to the lower face of the longitudinally plicate inwardly 

 extended nephridial lamina. Tlie stem containing the main venous 

 trunk is not perceptible from below, except under magnification, when 

 a parting between the surface crenulations of each lateral portion is 

 visible, but very inconspicuous. The Avhole of the gill except the stem 

 is of extreme thinness, like a canopy of lace, and the portions on each 

 side of the stem are full and irregularly penduloug. There is no verti- 

 cal gill lamina, but the canopy, between its attached edges, bulges 

 downward in an irregular longitudinal prominence, as if not drawn suf- 

 ficiently taut. By careful scrutiny on the prominence corresponding 

 to the inner lamina a very slender longitudinal raphe may be detected, 

 probably corres])onding to the morphologically lower edge of the inner 

 reflected lamina of the gill in normal ctenidia. On the outer promi- 

 nence corresponding to the outer lamina there is no raphe. The lower 

 or respiratory surface of the gill resembles in miniature that of the 

 so called Turkish bath towel, the transverse lamellae being disposed 

 in rather regular zigzag, extremely minute, elevated bauds, frequently 

 interrupted, but often continuous across the whole lamina. The edge 

 next the foot is defended by a narrow membranous margin, which is 

 firmly attached to the median line of the foot behind, but only closely 

 appressed elsewhere. The stem of each gill curves round behind the 

 foot about midway of thecteuidial surface, and the two are joined in 



