584 



MOLLUSCA 



SUB-KIXGDOM VI 



Family 20. Spiroceratidae. Phylogerontic, uncoiled, and straight forms, 

 probably derived from some of the preceding family. Young are attenuated cones 

 with single straight costae, but have four rows of tubercles at an early stage and a 

 A concave zone along the venter. 



Sutures of Reineckian. type, 1 un- 

 reduced to the phylogerontic formula 

 of six lobes. 



Spiroceras, Quenst. (Fig. 12-2-2); 

 Apsorroceras, gen. iiov. Type A. 

 (Ham.) baculatus, Quenst. sp. In- 

 ferior Oolite. Incertae sedis: 

 Baculina, d'Orb. Oxfordian and 

 Neocomian. 



Family 21. Hoplitidae. Dis- 

 coidal and involute forms with costae 

 bifurcated on the sides at umbilical 





FIG. 1222. 



Spiroceras Mfurcatum, Quenst. sp. Upper Dogger (Cal- 

 lovian); Ehningeii, Wiirtemberg. A, Shell with protoconch 

 broken away, Vi- J>, Portion of venter. C, Suture-line. 



Fie. 1223. 



U<.>i>Ules tuberciilal-us, Sowb. 

 Gault; Folkestone, Englai 

 Siphuncle broken away. 



shoulders ; prominent tubercles at their forks, and also at or near their ventral 

 termini, these last being separated by a median zone or deep channel. Young of 

 some species have costae continuous across the venter, and resemble those of Sonneraf/n. 

 Parallelism with Cosmoceratidae very close. Sutures resemble those of Mammitidae, 

 but more complex. Lateral saddles narrower and more deeply cut, and first lateral 

 saddles often trifid in late stages. Dorsal series with two pairs of complex zygous 

 lobes and saddles on either side of a long, narrow, complex, antisiphonal lobe. 



Hoplites, Neum. (Fig. 1223); Genomanites, Haug (Discoceras, Kossmat) ; Son- 

 neratia, Bayle ; Anahoplites, gen. nov. Type A. (Amm.) splendens, d'Orb. sp. 

 Cretaceous. 



VIII. PLACENTICERATIDA. 



Young, smooth, an 1 compressed, with flat or concave venter, which may subsequently 

 become acute, or remain flat throughout life, or may even become keeled. Outline apt to lie 

 rounded in old age; sides may have as many as three rows of tubercles, but in most 

 forms are not highly ornamented. Sutures remarkably complex in some families, but in 

 others extremely simple, recalling those of Triassic genera. General tendency is to extend 



