42S .Mr. H. Htlicricl<>;e ott (\irhoinfcrouf! Lame.Uihrotichiata. 



? Mwliolu. sp., l{hind, .\«j:e of tlu' I^irtli. 18;5H, p. 107, t. 2. f. // (without 

 ilescriptiou). 



Mi/ii/its {Mi/ali))(i) rrffssn, King, Mdii. rorni. Foss. I'-ngl. 18-")0. p. IT)'.* 

 (without dosoription"). 



Mytilug (Tii.tsii.i, Morris, Cat. I^rit. Foas. 1854, p. 214 (without de- 

 scription). 



Mi/alitw rrassn, Huxley & Etheridge, Cat. Fo.<58. M«s. Pract. Geol. 1866, 

 p. 110 (without description). 



Mi/(ili)ui croMa, Armstruug \' Younjr, Cat. Carh. Foss. W. Scotland, 

 Trans. Geol. Soc. Glasgow, 1871, iii. Supp. p. /)2 (without description). 



Spec. char. Shell elongate, obliquely subtrigonal, very thick, 

 with strongly marked unihonal ridges, inoqui valve, large 

 whei\ full-grown. Anterior entl forming a small lobe in 

 front of the beaks and iiml)onal ridges ; its margin siinious, 

 and a little concave in outline. Posterior end compressed 

 in comparison with the anterior ; its margin slightly sig- 

 moidal in outline. Hinge-line straight, nearly equal to 

 the length of the shell. Beaks not quite terminal, small 

 and pomted, a little incurved at their apices. Umbonal 

 ridges prominent, extending from the beaks backwards and 

 downwards to o])posite the anterior part of the ventral margin, 

 Avhere they become lost in the general surface of the shell ; 

 they are most prominent immediately below the umbonal 

 region, and give to the valves a carinate appearance. Hinge- 

 plate broad, with the inner margin a little thickened, and 

 longitudinally marked by a very large number of cartilage- 

 furrows. Byssal sinus in the right valve more develo])ed in 

 some individuals than in others. Anterior adductor impres- 

 sion double, pit-like, and deep, placed within the umbonal 

 cavity. Posterior adductor impression large and transversely 

 elongated. Pallial line well marked in old individuals by a 

 series of pits along its course ; remote from the margin of the 

 valves. Shell-substance very thick, especially in the umbonal 

 region and cardinal area. Surface covered with an epidermal 

 investment, ornamented by close fine stria^. 



Obs. M. crassa was in the first instance referred to by the 

 late Rev. Prof. Fleming as a Modiola in his paper on the 

 Testaceous Annelides, as quoted above, but was afterwards 

 briefly described by him as a fossil species of the genus 

 Mytlhcs. With this short description, and a figure of a 

 Modiola in Rhind's ' Age of the Earth,' from the water of 

 Leith, near Woodhall (where it occurs in some abundance), 

 which I believe to be the same shell, the early history of M. 

 crassa is completed, so far as the literature of the subject is 

 known to me. The greatest number of cartilage-furrows I 

 have counted on the hinge-plate of any one specimen is 

 twenty-four. These grooves are apparently continuons round 



