108 Dr. F. A. Bather— The Lithodomons Worm Pohjdora. 



III. — PossiL Representativks of the Lithodomous "Worm Polydora. 



By Dr. F. A. Batheii, M.A., F.G.S., Brit. Mus. (Nat. Hist.). 



'■pHE borings of the Polychfet Annelid Polydora ciliata (Johnston)^ 

 JL are familiar in limestone pebbles and the stouter mollusc-shells 

 on our coasts, but shales and occasionally other rocks are not free from 

 their attacks. They may be recognized by the double aperture, often 

 loosely described as keyhole-shaped, leading to a U-shaped cylindrical 

 tube. The union of the two apertures in a single depression, by the 

 loss of the organically formed septum between them, produces a more 

 slit-like opening, whereas if the surface of the pebble be further worn 

 the two round holes are more clearly distinguished. The differences 

 between the burrows of Polydora and those of other lithodomous 

 Annelids, especially the associated Dodecaeeria, were clearly stated 

 by Dr. W. C. M'Intosh in a very full paper " On the Boring of 

 certain Annelids" (1868, Ann. Mag. jSTat. Hist. [4], ii, pp. 276-95, 

 pis. xviii-xx). 



The disintegrating action of these worms has been emphasized and 

 its geological importance pointed out by sevei'al writers, among whom 

 may be mentioned the Abbe Dicquemare (1781, Rozier's Obs. et Mem. 

 Phys., Paris, pp. 222-4, xviii) ; Mr. (now Sir) E. Eay Lankester 

 (1868, Ann. Mag. ISTat. Hist. [4], i, pp. 233-8, pi. xi) ; and Professor 

 L. Yaillant (1891, Ann. Sci. Nat. [7], xii, pp. 39-50). It would, 

 therefore, be of some interest to trace the palaeontological history of 

 this lithodomous Annelid. According to the revisions by Di'. D. Carazzi 

 (1893, Mitt. Zool. Stat. Neapel, xi, pp. 4-45, pi. ii) and by Mr. M. P. 

 Mesnil (C. P. Acad. Sci., Paris, cxvii, pp. 643-5), there are from ten 

 to fifteen species now living, but that which is of most importance as 

 a borer, at least in north-western Europe, appears to be P. ciliata. 

 This lives in the lower half of the littoral or, more strictly speaking, 

 inter-tidal region, and its forsaken burrows may therefore throw light 

 on the elevation or depression of the land. The existence of other 

 species must, however, guard us against rashly referring to this species 

 fossil borings of similar character. 



Though, so far as I can ascertain, this genus has not hitherto been 

 recorded in a fossil state, the collection of fossil Annelids in the 

 British Museum enables us to carry the history of P. ciliata or an 

 allied species back to the middle of the Pliocene. The following 

 examples may be mentioned : — Borings in a valve of Volsella modiolus 

 (Linn.), from the Glacial beds of the Clyde series ; the borings fringe 

 an area from which the outer layers of the shell have been broken 

 [A 235].^ A phosphatic nodule from the lied Crag of Suffolk, cut 

 and polished and showing numerous tubes ; in this the two limbs of 

 the U are in many cases cut across, and 6ach pair is enclosed by 

 concentric layers of the concretion, forming a sort of figure of 8 

 [58197]. A similar nodule from Woodbridge, Sufi'olk [58220]. 



Samuel Woodward, on p. 20 of his " Outline of the Geology of 



1 I have to thank my colleague, Dr. W. T. Caiman, for kindly referring me to 

 this species and some recent papers on it. 



- The relations of Polydora to recent oysters have been fully discussed by Professor 

 M'Intosh (1902, Ann. Mag. Nat. Hist. [7], ix, pp. 299-308). 



