218 



ON THE ANATOMY OF TAGELUS GIB BUS AND T. BIVISVS. 

 By H. H. Bloomer. 



Read lith December, 1906. 

 PLATE XIX. 



I DESIRE first of all to thank Professor W. H. Dall, of the United 

 States ^N'ational Museum, for so kindly allowing me to examine the 

 following specimens : — 



Tagelus gibbus (Spongier). 



External Characters. — The animal measures from the anterior side 

 of the anterior adductor muscle to the posterior side of the posterior 

 adductor muscle 48 mm. and is 19 ram. deep. It curves a little 

 dorsally along the dorsal surface, and laterally is slightly constricted. 

 Separately the mantle-lobes (Fig. 1, If.L.) project a little dorso- 

 anteriorly from the anterior adductor muscle, and passing around the 

 mantle edges it is observed that they are curved anteriorly, and there 

 is no conci'escence of the apposed surfaces until a position directly 

 ventral to the posterior adductor muscle is reached, which also marks 

 the great extent of the pedal aperture. Between this point of 

 concrescence and the postei'ior adductor muscle the mantle edges are 

 not again concresced. At the junction of the mantle-lobes, ventrally 

 to the posterior adductor muscle, is the musculus cruciformis (Fig. 1, 

 M.C.), the terminations of which are on each side, situated equally, 

 though some distance, apart. The proximal portion of the siphon 

 (Fig. 1, Ex.S. and In. 8.) commences close to the posterior adductor 

 muscle, but soon takes a deep bend inwardly, so that the central part 

 lies ventrally to the retractor pedis posterior muscle. It then 

 returns and ends above the musculus cruciformis. Along the antero- 

 lateral margin the siphon is coalesced with the inside walls of the 

 mantle-lobes, thereby completely enclosing the posterior part of the 

 pallial cavity, with the exception of the siphonal apertures. The 

 mantle-lobes extend a considerable distance posterior to their fusion 

 with the proximal portion of the siphon, and are capable of enveloping 

 the greater part of the free siphonal tubes (Fig. 1, P.L.P.). The 

 latter are long and separate, the exhalent (Fig. 1, E.c.S'.) being longer 

 than the inhalent one (Fig. 1, In.S'.). They have thick walls and 

 are very muscular. Internally they are ribbed longitudinally and 

 transversely, the transverse ribbings being very close together. The 

 retractor siphonal muscles (Fig. 1, S.E.M.) are short and thick, and 

 soon spread out into an oblong- ovate -shaped body, and from a large 

 surface adhere to the valves of the shell. 



When contrasted with Tagehis rufus ' it will be observed that in 



' The Anatomy of Pharelta orientalis, Dunker, and Tagelus rufus, Spengler " 

 Journ. Malac, vol. x (1903), p. 114. 



