166 SILURIAN TRILOBITES. 



pleurae are flat as far as the fulcrum, truncate at their ends, and have but a slight groove, 

 which reaches only two thirds of the length. The fulcrum is at one third in front, and 

 less than half-way out in the middle rings. 



The caudal axis extends three fourths down the smooth tail, very indistinctly marked 

 above, but in some specimens crossed by several faint rings, and is always prominent at 

 the tip (see figs. 10, 11). 



This has the characteristic facial suture of Isotelus ; but if its labrum be like that of 

 A. affinis, a point which is not yet known, it may belong to quite a distinct subgenus. 



A. Homfrayi differs from its associate A. affinis, not merely in the facial suture, but 

 in the much less pronounced axis of the tail ; and from A. Powisii, vdth which it might 

 be confounded by casual observers, the want of any tail-furrows, except obscure ones, on 

 the axis, and the very slight convexity of the glabella, will easily separate it. Besides, it 

 grew to only half the size of that Caradoc species. 



Localities. — Upper Tremadoc Slate. Under Garth, near Tremadoc. Railway 

 behind Penmorfa village. Tu-hwnt-yr-bwlch, near Portmadoc (cabinet of Messrs. Ash 

 and Homfray, and Mus. Pract. Geol.). 



$ Brachtaspis, Salter, 1866. 



AsAPHUs (Isotelus) rectierons, Portlock} PI. XXV, figs. 6 — 10. 



Isotelus kectiprons, Portlock. Geol. Rep., p. 298, pi. ix, fig. 1, a, h, and pi. viii, 

 figs. 2, 3, 7 (head), 1843. 



— ARCUATUS, Id. lb., pi. ix, figs. 2, 3 (tail). 



— RECTiFRONS, Salter. Mem. Geol. Surv., Decades No. xi, sec. 3, p. 4, 1864. 

 [Compare A. IcBvigatus, Angelin, Pal. Suecica, t. 29, fig. 1, 1852.] 



A. {Brack.) late ovatus, lavis, obttisus, cui caput caudaque transversa, convexa sunt. 

 Caput breve, vix setnicirculatum, angulis obtusis deflexis. Oculi arcuati, procul pone 

 medium caput positi. Sutur a facialis postica dimidium genes efficit, anterior axi parallela est. 

 Thoracis axis latus, vix a pleuris arcuatis distinctus, fulcro propinquo. Cauda brevis 

 {long : lat :: 7 : 12), axe lato, 11 annulis minime pro/undis distinctus, /laud prominente, sed 

 in apicem prominulum desinente : latera, nisi sulco superno lato exarata, cequalia, limbumque 

 angustum planiorem gerentia. Fascia modica, striis distantibus. 



I venture to consider this the type of a distinct subgenus, intermediate between 

 Isotelus and Cryptonymus ; the extremely short form of the head and tail, and the 

 marginal suture in front, distinguishing it easily. A. lavigatus, Angelin, figured in the 

 next page, belongs to the same group. 



1 General Portlock himself said that the species should form a new group, intermediate between 

 Asaphus proper and Isotelus. 



