F. E. C. Heed — Haverfordwest Fossils. 435 



high, incurved, rising well over that of opposite valve, with small 

 triangular delthyrium below it. Hinge-line gently arched ; cardinal 

 extremities rounded ; no distinct false cardinal area. Teeth large, stout, 

 short ; no dental lamellae. Posterior and umbonal part of valve much 

 thickened internally with deeply impressed trilobed muscle-scar, 

 composed of short, broad, median, double groove in beak, bifurcating 

 anteriorly into two elongated, narrow, pointed diductor scars diverging 

 at 90°-120°, extending about one-third the length of the valve, 

 dying out towards extremities, with a somewhat elevated crescentic 

 area in the fork between them. 



Brachial valve less convex than opposite valve, rarely with traces of 

 weak median fold near anterior margin ; umbo small, inconspicuous. 

 Hinge-plate prominent, stout, divided medianly by narrow slit into 

 two large subtriangular rounded lobes with vertical flattened faces, 

 each bearing a submedian, small, sharp, crural process on a level with 

 or slightly behind the dental sockets, and projecting obliquely forwards 

 and downwards. Dental sockets deep. Median septum present, 

 extending about half length of valve, thick at base, decreasing in 

 height and thickness anteriorly. Muscle-scars weakly impressed, 

 elongated, narrow, lying alongside of septum. Surface of valves 

 marked by 150-200 narrow, low, rounded, closely-set, regular, simple 

 ribs, of equal size, and not increasing by division or interpolation 

 near margins; crossed by very fine concentric lines and a few rare 

 growth-striae. 



Dimensions. — Average length 9-10 mm., width 11-12 mm. 



Horizon and Locality. — Llandovery Beds, The Frolic, Haverfordwest. 



Remarks. — The specimens occur in places in crowds, closely massed 

 together, and are found as internal casts and external impressions of 

 the shell. The external and internal characters of this Brachiopod 

 much resemble the form described by Davidson ^ as Rhynchonella (?) 

 pentlandica (Haswell), from the Ludlow of the Pentland Hills, and 

 they are probably congeneric. The muscle-scars and teeth of the 

 pedicle-valve and the medium septum and hinge-plate of the brachial 

 valve seem modelled on the same plan, but the division of the hinge- 

 plate and the presence of crural points are well marked in the 

 Haverfordwest species, and the latter also lacks the longitudinal 

 groove of the pedicle valve, and the ribs do not increase by inter- 

 polation near the margins. The shell also has normally a more 

 transverse shape. The similarity of the muscle-scars in the pedicle 

 valve to those in Dayia navicula (Sow.) is noteworthy. In external 

 characters and ornamentation Zygospira {Catazyga) Headi (Billings)'^ 

 may be compared, but the internal features more resemble typical 

 members of the genus Zygospira, and to this genus it may be 

 provisionally referred. It is distinguished from Z. Hichsi, Eeed,^ of 

 the Slade Beds of Cuckoo Grove Lane, and Upper Slade, Haverford- 

 west, by its fine ribbing, and the different character of the muscular 

 impressions in the pedicle- valve. 



1 Davidson, Mon. Brit. Bracli., iii, p. 187, pi. xxii, figs. 9-18a (? figs. IQa, b). 



2 Hall & Clarke, Palseont N.Y., viii, Brach. ii, p. 157, pi. liv, figs. 24-34. 



3 Eeed, Geol. Mao., Dec. V, Vol. II (1905), p. 452, PI. XXIII, Figs. 17-19. 



