F. R. CoR-per Reed — On the Cheiruridce. 209 



lobation of the glabella is also distinct, and this character, together 

 with the presence of the ridge on the pleura, is considered by Barrande 

 to unite the genus with the genera Cheirurus and Sphcerexoclius. 

 But the former character is a larval one, retained to maturity in 

 iTiany utterly distinct genera, and the pleural ridge is shared by 

 JSncrinurus. The hypostome is entirely different to that of Cheirurus. 

 The number of the thoracic segments (11-12), though the same as 

 that of some species of Cheirurus, is in itself an almost valueless 

 character. The pygidium alone is a treacherous guide, but its 

 general resemblance to some of the Eccopiocheile group is worthy 

 of notice. In PI. Zippei there are live fings on the axis, but in 

 PZ. Tournemini only four. Instability in the number of segments 

 in the pygidium and thorax is generally characteristic of the less 

 highly specialized forms. 



It does not seem possible to establish any definite line of ancestry 

 or affinity by which we may link Placoparia with other known 

 genera ; we must thei'efore regard it as an aberrant member of 

 tlie family, diverging at an early period from the main stock, and not 

 directly giving rise to any of the later forms. 



The genus Amphion possesses a somewhat specialized head-shield, 

 for the first side-furrows on the glabella are shifted forwards to the 

 anterior border, and a small supplementary central furrow appears 

 in some species between them. The presence of an epistome and 

 pits on the cheeks are, as Schmidt says, points of resemblance to 

 Cheirurus, but these characters are of different values. Eyes, though 

 small, exist on the free cheeks, and the course of the facial suture is 

 similar to that in Sphcerexochus. The large and variable number of 

 segments (14-18) in the thorax appears to indicate that the genus was 

 still suffering evolution and was in a plastic condition, and the large 

 number of body-rings alone is usually considered as a primitive feature. 

 Thus Beecher has said that the indefinite multiplication of segments 

 is to be regarded as a primitive character, expressive of an annelidan 

 style of growth. On the other hand, the ridged pleurae and the 

 absence of any furrow on their surface recall Sphcerexochus, and are 

 apparently non-primitive features. The pygidium, with six rings 

 on its axis and five pairs of pleurae with free ends (of which the two 

 last may be fused into one plate, as in Amphion pseudo-articulafus), 

 reminds us of the multisegmented pygidia of some Encrinuridee, and 

 does Bot show any points specially associated with the Cheirurid^. 

 Barrande placed this genus with the Encrinuridas chiefly on account 

 of its numerous body-segments, but from the above review of its 

 main characters the evidence for its location amongst the Cheirui'idas 

 appears to be stronger. It is clear, however, that it branched off 

 from the main stem at an early period, and pursued a somewhat 

 independent line of development and specialization. Its strati- 

 graphical antiquity also leads us to place its divergence at an 

 early date. 



The Russian genus Biaphanometopus^ seems allied to it; the 



1 Mem. Acad. Imp. Sci. St. Petersb., vol. xxs, Ko. 1 (18S1) : Eev. ostbalt. Tril., 

 Abth. i, p. 195. 



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