8 ARETHUSA. 
APOROBRANCHIATA. Bl. The first order of the second 
section of Paracephalophora Monoica, Bl. The Thecosomata 
is the only family of this order containing any appa to 
shells, these are Hyalzea and Cymbulia. 
APLY'SIA. Lam. (a, without; Mvo, pluo, to wash.) Fam. La- 
plysiens, Lam. Aplysiacea, Bl. Deser. Horny, transparent, cly- 
peiform, placed horizontally on the back of the animal, with its 
convex side uppermost; apex slightly incurved. Oés. The animal 
producing this shell has derived its name from the purplish 
liquor which it exudes, when disturbed. In contour it presents 
a certain likeness to a hare crouching, and on this account was 
called Lepus marinus, or sea hare, by the ancients. The shell 
bears a strong resemblance to Dolabella, which however is much 
thicker and more testaceous. A. Petersoni, fig. 254. 
APORRHA'S. Petiver. See Rosrettaria. <A. Pes-pelicani, 
f. 404. 
AQUIL'LUS. Montf. Triton Lampas, Cutaceus, &c. Auct. 
Placed by Blainville in the division of the genus Triton, which 
is described as having a short spire, being covered with tubercles, 
and umbilicated. Triton Cutaceus, fig. 399. 
ARCA. Lam. (Lat. a boat.) Fam. Arcacea, Lam. JDescr. 
Obliquely transverse, subquadrate, equivalve, or nearly so, 
ineequilateral, thick, ventricose, longitudinally ribbed, dentated 
near the inner margins; hinge rectilinear, forming a flat exter- 
nal area, upon which the ligament is spread in transverse rows, 
and having a series of small regular teeth, extending on both 
sides of the umbones in each valve; muscular impressions 
distant. These shells are found recent, in various marine locali- 
ties, fossil, in the tertiary deposits. The Arca Noe, formerly 
regarded as the type of this genus, has, with several other 
species, been separated from it under the name Bysso-arca, by 
Swainson, on account of an hiatus in the ventral margin to 
admit the passage of a byssus; this is not found in true Arce. 
Arca Antiquata, fig. 131. Bysso-arca Noe, 132. 
ARCA’CEA. Lam. A family of the order Conchifera Dimy- 
aria. Lam. Consisting of bivalve shells, provided with a 
linear series of small teeth on the hinge. In this family 
Lamarck describes the genera Cucullea, Arca, Pectunculus, and 
Nucula, to which may perhaps be added, Crenella, Solenella, 
Sow. and Myopara, Lea. Fig. 131 to 138. 
ARCHA‘TAS. Montf. <A genus of microscopic Foraminifera. 
ARCHON’TE. Montf. Hyauma, Auct. 
ARCINELLA. Schum. Cuama Arcinella, Auct. 
ARC’TICA. Schum. Cypriva Icelandica, Auct. 
ARCUATED. (Arcus, anarch.) Bent in the form of an arch, 
as Dentalium, pl. 1. 
ARETHU’SA. Montf. A genus of microscopic Foraminifera. 
