ll 
ADDENDA. 103 
Genus—Onvstus, Humphrey. 
Shell conical, with several volutions, which are flattened or are rendered somewhat 
concave by an expansion of their lower borders, which overhang and conceal the suture ; 
the lower border of the last volution is produced horizontally to support a membranous 
expansion ; the surface has striations, or radiately undulating lines, which are somewhat 
irregular. ‘The base is concave towards the outer border, and convex towards the centre ; 
the umbilical orifice is sometimes large, but in other instances small, and becomes nearly 
concealed by advance of growth; the aperture is depressed and ovate. Some Tertiary and 
Recent species have the spire encrusted with fragments of shells or stones, which obscure 
the ornamentation. 
Xenophora, Fischer, and Phorus, Montfort, are synonyms of this genus. 
Onvstus Burtonensis, Lye. Tab. XLV, figs. 7, 7 a, 4. 
Testa subconica, spira elevata, obtusa, anfractibus (4-5), angustis subconcavis, longi- 
tudinaliter costatis, costis (circa 24—26) inferne alternatim in spinis producta ; basi sub- 
concavo, concentrice et radiatim striato, umbilico amplo. 
Shell subconical, wider than high; spire moderately elevated obtuse; volutions four 
or five, narrow, slightly concave, with about twenty-four to twenty-six longitudinal rounded 
and elevated coste ; the base of every alternate costa forms, with the lower expanded mar- 
gin of each volution, a projecting process, which renders the lower margins of the volu- 
tions undulated; the base is expanded, slightly concave, concentrically and radiately 
striated ; the umbilicus is large. 
A pretty species, possessing the generic features strongly defined, more especially the 
expansions at the lower border of each volution, which impart a pagoda-like aspect to the 
spire. Only two other British Jurassic species are known, viz , Zrochus pyramidatus, Phil., 
= Trochus lamellosus D’Orb., a more depressed species, which occurs in the Supra-Liassic 
sands, and in the Inferior Oolite of Gloucestershire and of Yorkshire; the other is the 
Trochus ornatissimus, D’Orb., with a very elevated spire, and inordinately expanded at 
the lower border; it occurs in the Inferior Oolite of the Cotteswolds, and in the White 
Oolite of Ponton, Lincolnshire. Our species is most nearly allied to Zrochus ornatissimus, 
but with a shorter spire, fewer volutions, and with prominent overwrapping expansions at 
the lower border of each volution. Other foreign Jurassic species are Zrochus heliacus, 
D’Orb., 7. Zytirus, D’Orb., Solarium callaudianum, D’Orb., Onustus exul, Eug. Desl., 
and Onustus liasinus, B. Desl. None of these species exhibit those agglutinations of 
shells and stones which are so characteristic of the Tertiary and Recent examples of 
Onustus. 
