F. R. Cowper Reed — Fossils from Haverfordwest. 499 



Dimensions. 



I. II. 



Length 15"0 ... 15-Onim. 



Height at beak ... ... ... 4*5 ... .5-Omtn. 



Height at posterior end ... ... 7 "5 ... 7'Omni. 



Horizon and Locality. — Slade Beds : roadside near St. Martin's 

 ■Cemetery, Haverfordwest. 



Remarks. — The above described species varies somevirbafc in its 

 proportions, but is certainly distinct from the British species 

 previously described from Ordovician rocks which can be referred 

 to the genus Modiolopsis in the restricted sense in which the term is 

 used by Ulrich (op. cit., p. 502). The definition given by him is the 

 following : — " Shell more or less elongate, usually subovate, widest 

 posteriorly; valves moderately ventricose, closing tightly all around. 

 Beaks small, near tlie anterior extremity ; umbones depressed by 

 a flattening or depi-ession which crosses the valves obliquely, and 

 widening causes a straightening or sinuation of the basal outline. 

 Hinge of moderate strength, rarely straight, generally somewhat 

 arcuate, without well-marked teeth ; an obscure oblique thickening 

 ■beneath the beak of one valve and a corresponding depression in the 

 other occasionally distinguishable. Ligaments linear, external and 

 internal, chiefly the former. Anterior adductor impression sub- 

 ovate, large, deep, sharply defined on the inner side, occupying the 

 greater part of the small anterior end. Posterior scar very faintly 

 impressed, large, subcircular, situated near the centre of the posterior 

 third of the cardinal slope. Pallial line simple. Anterior pedal 

 muscle forming a minute pit in the under side of the hinge- 

 plate beneath the beak. Posterior pedal muscles large, attached 

 just above and in front of the adductor. Type : M. modiolaris, 

 Conrad, sp." 



The Slade species appears to be allied to M. arguta, Ulrich 

 (op. cit., p. 506, pi. xxxvi, figs. 3-6), and M. nana, Ulrich (op. cit., 

 p. 507, pi. xxxvi, fig. 7), from the Trenton and Galena Shales. 

 The British shells attributed to 3f. modiolaris, Conr., and M. Nilsoni, 

 His., sp,, are much shorter and higher, and have a broader anterior 

 end. Salter's Mytilus gradatus ^ is likewise a less elongated form. 

 Portlock's Modiola expansa' from Desertcreat is allied, but un- 

 doubtedly distinct from our species. 



There is a variety of M. Martini occurring in association with 

 the typical form, and scarcely separable from it specifically; it is 

 •characterised by possessing a less elongated shape, a smaller 

 widening of the posterior end, a somewhat broader anterior 

 extremity, and rather stronger varices of growth on the surface. 

 The specimens are mostly not well preserved, and intermediate 

 forms connecting it with the type are not uncommon. Some 

 specimens resemble the longer forms of M. gradata, Salter (op. cit., 

 figs. 3 and 4), but the latter occurs in the Ludlow Beds, and is 

 probably distinct. 



1 Salter: Mem. Geol.'Surv;, vol. ii, pt. 1, p. 363, pi. xx, figs. 3-5. 

 - Portlock : Geo!. Hep, Londond., p. 425, pi. xxxiii, tig. 6. 



