398 CRETACEOUS GASTROPODA 
opERCULATINE (= Umusrettipa, auctorum), including Operculatum and Tylodina, 
there are many species known from the Silurian deposits upwards. Of cretaceous 
shells I may draw attention to the genus Anisomyon of Meek and Hayden, which 
I have mentioned, p. 325, in the family Zzpzrrpz. The shells are very closely 
allied to Tylodina, being thin, patelliform, with a pointed, recurved apex, sup- 
posed to be perforated. Gray (Guide, 1857, p. 203), however, says that the 
shell of Zylodina has a globular, spiral and sinistral nucleus and the muscular 
impression with a sinus on the right side, while in Anisomyon the muscular 
impression is said to be interrupted in front above the head, as in Lepeta, 
LVI. Family,—ACTHAONIDA. 
The animals of the Acr#owrp# have been observed only in very few instances. 
Though generally perfectly retractile, they are mostly of large size when expanded, _ 
having a large thick foot, reflexed on the sides; head broad, very often forming 
a flat disk, with or without other appendages ; the tentacles are broad, thick, united 
at the base, but generally separated at their tips, the eyes being sessile above near 
their base. The teeth are numerous, uniform, arranged in diverging cross series, 
the central ones are wanting. 
Operculum, when present, horny, ovate, narrow, composed of few whorls. 
The shell is spiral, ovate, convolute or mvolute, with more or less elevated spire, 
mostly spirally punctuated, the aperture is usually high and narrow, truncate or 
roundish in front, the columella usually solid. 
Important characters of the shell of the dcrmonrp# are the large size of the 
last volution and its usual spiral punctuation, which appears to be only very rarely 
wanting. Meek’s last monographic exposition* of the family is in many respects 
very imperfect, and, notwithstanding the great desire of introducing new genera, 
the author has omitted many which were already known; he could scarcely have 
examined any of the fossil forms, otherwise I think that he could not have 
regarded some of the most unimportant and accidentalt distinctions as of generic 
value. 
a. Sub-family,—ACTAONIN 4. 
Shell ovate, aperture anteriorly rounded, sometimes broadly effuse, outer lip 
sharpened at the margin, columellar lip twisted and often plicated in front ; the 
recent species have an operculum. 
This is the largest and oldest group of orxonzps, the first forms not pos- 
sessing a columellar fold, probably beginning already in the Silurian ; at least some 
of the species described by Eichwald very much resemble Acteonina. From the 
earboniferous beds the Chemnitzia carbonaria, Koninck, and others are known to 
belong to Acteonina. In the Trias the species become more numerous, and some of 
them already very closely resemble recent forms of Acton. Other species are found 
* American Journal of Sc. and Arts, XXXV, 1863, p. 89, etc. 
+ Being sometimes only dependent upon the state of preservation of the shell. 
