PYRGOMA MILLfiPORiE. 367 



said to exist any spur, the whole basal margin being almost straight; 

 nevertheless, on close examination, the ridge which in appearance sepa- 

 rate the scutum and tergum, may, I think, be safely considered as one 

 side of the spur (which, it should be remembered, has in all ordinary 

 cases a longitudinal furrow or depression), and the other side of the 

 spur is, apparently, very feebly indicated by a slight flexure in the 

 middle of the basal margin. Hence, if the spur had been developed, 

 it would probably have been half as wide as the valve. There are no 

 crests for the tergal depressor muscles. 



Affinities. — The present species, with the last two, form an interest- 

 ing series. Pyrgoma grande and conjugatum, however, are more closely 

 allied to each other than to P. eancellatum. In the scutum, the whole 

 valve is least elongated, with the adductor plate least developed, in P. 

 grande, and most elongated, with the adductor plate most developed, 

 in P. eancellatum. In the outline of the tergum the range of shape is 

 quite remarkable ; in P. conjugation, which stands between the other 

 two species, the spur is rather long, whereas in P. grande there is no 

 spur at all — a very unusual circumstance — and in P. eancellatum, at 

 the other end of this short series, the spur attains a length wholly 

 unparalleled in any other cirripede. 



6. PYRGOMA MILLEPORiE.* PI. 13, fig. 2 a 2f. 



SJiett with the orifice narroidy ovate : sheath dark purple : 

 scutum much elongated: tergum triangular, convex, ivithout a 

 spur. 



Bab. — Philippine Archipelago (Mindoro Island), Mus. Cuming. Mus. Brit., 

 Stutchbury, &c. Imbedded in Millepora complanata, sometimes associated 

 with Balanus Ajax. 



Appearance and Structure of Shell. — Shell oval, flat, coloured pale 

 dull purple, or white, with slight and narrow ridges, radiating, from the 

 orifice, which is not quite central, but is placed rather nearer to the 

 carinal than to the rostral end of the shell. The orifice is small and 

 narrow ; the carinal end being rounded, and the rostral end narrower 

 and sharper, — this being the exact reverse of the usual shape of the 



* From external aspect I suspect this species to be the Creusia madreporamm, 

 Leach (?), as given in Chenu, 'Illust. Conch./ Tab. 1, fig. 6. But I feel sure 

 that Leach has nowhere published this name ; and it may be observed that Chenu 

 gives it with a mark cf doubt. The shell in its imbedded state is only figured ; 

 the opercular valves are not given ; and no descriptive details are added. Under 

 these circumstances I have not adopted this name ; I have, perhaps, been in 

 some degree influenced by the fact that this species, judging from the many 

 specimens examined by me, is never imbedded in madrepores, but exclusively 

 in millepores, so that Chenu or Leach's specific name of Madreporarum is 

 singularly inappropriate. 



