688 



MOLLUSCA— CEPHALOPODA 



PHYLUM VI 



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and calcareous layers, widest in front, rounded jjosteriorly. Median area divided into 

 halves by a longitudinal line, and bounded on either side by lateral areas with 

 ^1 hyperbolic striae. Ink-bag frequently preserved, the 



contents transformed into a jet -like substance. It is 

 jjossible to dissolve the carbonaceous particles so as to 

 prejiare a wash resembling India ink. Upi^er Lias of 

 Germany, France and England. 



Beloteihthis Miinst. (Fig. 1335), Proostracum very 

 thin, elongated, feather -shaped, broadly rounded pos- 

 teriorly, pointed in front, traversed by a median 

 longitudinal keel. Upper Lias of Wiirtemberg. 



Teuthopsis Desl. Lias. Kclaeno- Miinst. Upper 

 Jura. Phylloteuthis Meek and Hayden ; Actinosepia 

 Whiteaves. Cretaceous ; Canada. 



Plesioteuthis Wagner {Dorateuthis Crick) (Fig. 1336). 

 Proostracum very thin, long, narrow, lanceolate, pointed 

 posteriorly, rounded in front, with a median longi- 

 tudinal keel and a raised line along each of the lateral 

 edges. Very abundant in the Lithographic Stone, and 

 impressions of the liody and head not uncommon. Also 

 found in the Cretaceous of Maestricht and Syria. 



Order 3. OCTOPODA Leach. 



Body vnthout internal shell, and only the female of 

 Argonauta secreting a single- chavihered external shell. 

 The two tentacles are not present, and the eight arms 

 bear sessile suckers withoid horny rims. Eye relatively 

 small, ivithout sphincter -like lid. Body short and rounded, 

 usually without fin-like appendages. 



The majority of genera belonging here aie naked 

 and therefore without fossil representatives. The small 

 male of Argonauta Linn, is without a shell, but the 

 large female bears a delicate, boat -shaped, spiral shell 

 which is secreted partly by the mantle, and partly by 

 two fin-like expansions of the dorsal arms. Outer surface 

 of shell ornamented by folds and tubercles, and two 

 „,.,,. . ,„ . „ nodose ventral keels are present. Late Tertiary (Pied- 



Pleswtenlhis prisca (Ruppel). \ i -n j 



Lithographic Stone ; Eichstadt. mont) and Kecent. 



;'• I"'P7««ion "f aniinal showing q^^i^^ j | q g ^ g^ ^ ^ ^ j 



anus and ink-bag. B, 81iell, i/.,. ... . . 



provided with triangular lateral fins, not united behind. 

 Head small, with relatively stout tentacttlar arms, these being of nearly uniform 

 length and size, and each bearing a single row of suckers. This is the earliest known 

 Octopod genus. Upper Cretaceous ; Mt. Lebanon, Syria. 



Fi(i. 133(5. 



Vertical Range of the Dibranchiata. 



As compared with Tetrabranchiates, the Dibranchiata are of minor geological 

 importance. Their entire organisation renders them less well adapted for preserva- 

 tion in the fossil state, and accordingly we shall never be able to form even an 

 ajiproximate idea of their importance in their contemporaneous faunae. The earliest 

 representative of Belemnoidea ajspears in the Trias (Aulacoceras), and the Sepioidea 



