Dr. J. Lycett on a Byssiferous Fossil Trigonia. 17 



III. — On a Byssiferous Fossil Trigonia. 

 By John Lycett, M.D. 



The discovery of a byssal aperture in a fossil Trigonia, in 

 connexion with certain features which are presumed to have 

 been physically connected with such a condition of existence, 

 is a novelty in fossil zoology, and, as such, needs no apology, 

 although the species was figured and partially described up- 

 wards of twenty-eight years since. I allude to Trigonia cari- 

 wo.to, Agassiz, found in the Lower Greensand of various French 

 and English localities. The well-known memoir by Agassiz on 

 the genus Trigonia contains figures of this species represent- 

 ing merely immature casts, in which the ornamentation of the 

 surface is only very imperfectly indicated; and the description 

 also accords with such an unsatisfactory condition. D'Orbigny, 

 in his ' Paleontologie Fran^aise,' has given elaborate figures of 

 a single perfect specimen of adult or nearly adult growth. 

 Upon referring to plate 286 of the work last cited, we find a 

 marvellously perfect example of T. carinafa, possessing all the 

 usual sectional characters of the costata?, remarkable more 

 especially for the salient ornamentation of the area, with its 

 large carina? and intermediate costellte : these features, so 

 beautiful in the earlier stage of its growth, disappear altogether 

 in specimens that have attained to about half the dimensions 

 of adult shells, and are replaced by irregular, large, rugose, 

 ti-ansverse plications ; but in the figures of D'Orbigny we dis- 

 cover nothing of this : the area retains its pristine ornamenta- 

 tion unaltered to its ultimate stage of growth — a condition of 

 existence which we may never expect to discover in any 

 actual specimen. The same figures have no indication of a 

 byssal aperture, and the cost» have less than their real obli- 

 quity. The author's text is only a brief description of the 

 figures of the artist. 



The general figure of T. carinata is remarkable as com- 

 pared with examples of the genus generally ; it is oblong 

 or ovately oblong, much lengthened and narrow or inflated 

 along its mesial portion, and has in fact much genei-al resem- 

 blance to Byssoarca. The byssal aperture is not large, and is 

 placed at the anterior or antero-inferior border. An examina- 

 tion of the lines of growth shows that this orifice was formed 

 only when the valves approached to their adult condition. 

 Specimens which exhibit the complete or uninjured outline of 

 the valves are all of immature growth, and had not formed 

 the byssal aperture. Valves of adult growth are found in a 

 condition altogether different and in accordance with the al- 

 tered habits of the moUusk : the lively bivalve, with its salta- 



Ann.& Mag. N. Hist. Ser. 4. Vol.y. 2 



