44 U.S. NATIONAL MUSEUM BULLETIN 291 



is about 50% of the mantle length. The location of the mantle-funnel 

 locking apparatus is indicated ventrally on each side of the mantle 

 opening by a small triangular projection. The anterior projection on 

 the dorsum of the mantle is a blunt lobe. The muscle that makes up the 

 wall of the mantle is moderately thick and well developed. The in- 

 tegumentary layers are relatively thick, similar to those of B. dbys- 

 sicola; they are bound to the mantle wall by a semigelatinous, fibrous 

 matrix of connective tissue. 



The fins are short, small, and paddle-like ; they are nearly circular 

 in outline (pi. 6). The fins are subterminal; the bases are separated 

 posteriorly by the blunt end of the mantle and anteriorly by the broad 

 dorsal surface of the mantle. The bases of the fins are thick and muscu- 

 lar, but the margins are thin and fragile, almost membranous ; they are 

 easily torn so that the actual outline of the fins is difficult to deter- 

 mine. The anterior and posterior fin lobes project well beyond the 

 bases of the fins and the posterior lobes extend beyond the end of the 

 mantle. 



The funnel is broad at the base and tapers anteriorly (pis. 6b; 7a). 

 The exhalant opening extends nearly to the level between the posterior 

 margins of the eye openings. The posterior border along the base of 

 the funnel is deeply concave between the funnel components of the 

 locking apparatus. The funnel retractor muscles are strong and robust. 

 The bridles are weak, thin bands of muscle. 



The collar is a simple, thin-walled band of muscle that passes dor- 

 sally from the base of the funnel to the nuchal lock. 



The funnel and the collar are bound to the head by the integument 

 and the subcutaneous, semigelatinous connective tissue so that only 

 the anterior portion of the tube is free. The funnel groove is short and 

 relatively shallow. 



A small median orifice in the integumentary tissue occurs near the 

 posterior end of the funnel groove, dorsal to the exhalant tube (pi. 

 7r) . The orifice marks the opening of a narrow tube that passes dor- 

 sally through the gelatinous tissue and between the median edges of 

 the bridles. This unusual structure, which occurs in some other groups, 

 is currently being investigated. 



The funnel component of the locking apparatus is a long, narrow, 

 cartilaginous structure of the simple type (pi. 7a, b). The rounded 

 posterior end is broader than the rounded anterior end. The sulcus of 

 the component is smooth and relatively shallow ; it is slightly deeper 

 and narrower anteriorly. A narrow, membranous lip outlines the struc- 

 ture. The mantle component of the locking apparatus is a simple, low 

 ridge; it is highest and most pronounced anteriorly and diminishes 

 posteriorly (pi. 7a, c). 



