. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 478 BiiUeiin Museum of Comparative Zoology, Vol. 135, No. 9 mm W\-/}r/^A:yv-/2_. Figure 9. Leonaspis cf. williamsi Whittington, 1956. Major paired spines of a small cronidium, original of Plate 16, figures 12-14, lettered following Whittington, 1956c, text- 1. an early odontopleurid developmental .stage. These include pains 2 to 5 on the glabella, Ai^a on the fixed cheek, and the spine on the eye ridge (compare Whittington, 1956c, text-figs. 1, 6, 9, 22, for corresponding spines in develo]:)mental stages of o
. Bulletin of the Museum of Comparative Zoology at Harvard College. Zoology. 478 BiiUeiin Museum of Comparative Zoology, Vol. 135, No. 9 mm W\-/}r/^A:yv-/2_. Figure 9. Leonaspis cf. williamsi Whittington, 1956. Major paired spines of a small cronidium, original of Plate 16, figures 12-14, lettered following Whittington, 1956c, text- 1. an early odontopleurid developmental .stage. These include pains 2 to 5 on the glabella, Ai^a on the fixed cheek, and the spine on the eye ridge (compare Whittington, 1956c, text-figs. 1, 6, 9, 22, for corresponding spines in develo]:)mental stages of other genera). XANIONURUS n. gen. Type species. Xanionurus boucoti n. sp. Diagnosis: Differs from ))is in that: 1) lateral glal^ellar lobes are not fused, but separated by deep Ip furrow; 2) there is a distinct, shallow occipital furrow and the occipital ring is relatively much shorter (sag.); 3) second axial ring of pygidium low, faintly divided by median longitudinal furrow and bearing pair of spines; 4) border of pygidium with 14 (not 16) spines, the posterior band of the anterior segment running into the base of the 5th (not the 6th) border spine; 5) besides the main paired spines on the glabella there are many additional spines and granules; similarly, on the cheek there are spines on the upper surface of the bor- der, and many scattered spines and gran- ules as on the pygidium. In Radiaspis the exoskeleton is smooth between the main spines. Geological range. Upper Wenlock to Ludlow. Discussion. Bruton (personal communi- cation ) points out that Radiaspis is like Diacanthaspis in lacking major border spines on the pygidium and in the ra- diating arrangement of the border spines. We agree with this, and add that the early developmental stages of the cranidium of the two genera appear to be similar, and the thoracic segments are alike. As in Acidaspis and Dudleyaspis there is a stout posterior sutural ridge in the new genus and a steeply inclined row of spines on the l
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