2 GEOLOGICAL MEM0IK8. 



Phylloeeras Capitanei, Catullo. Middle Lias. 



Nilsoni, Heb. Upper Lias. 



conneotens, Zitt. Inferior Dogger. 



heteropliylloides, Opj). Middle Dogger. 



■ disputabile, Zitt. Upper Dogger and Kelloway group. 



— • — Demidoffi, Bouss. Of uncertain age (Crimea). 



Manfredi, Oj^j^. Oxford group. 



Puschi, Ojjp. Oxford group. 



benacense, Cat. Asjyidoceras-acanthicmn beds. 



Kochi, 0pp. Tithonian. 



3. Series of Pliylloceras idtramontcmum. — Kneed radial fur- 

 rows ; striae of shell entirely wanting, or only next to the outer 

 side, and then coarse and short ; lobes and saddles clumsy ; first la- 

 teral saddle of the inner side with two-leaved termination. Species : — - 



Phylloeeras ultramontanum, Zitt. Liferior Dogger. 



Zignodianum, ly Orb. Middle Dogger. 



mediterraneum, sp. n. Klaus beds and Inferior Tithonian. 



polyolcum, Ben. Aspidoceras-acanthieum beds. 



silesiacum, 0pp. Tithonian. 



Calypso, If Orb. (=?P. berriasense, B'Orb.). Neocomian. 



4. Series of Phylloeeras tatricmn. — Eounded prominence on the 

 external side ; radial striation very faint, or entirely deficient ; 

 saddles clumsy, first lateral saddle on the internal side (?). Species : — ■ 



Phylloeeras tatricum, Pusch. Inferior Dogger. 

 — — flabellatum, sp. n. Klaus beds. 



Hommairei, I)' Orb. Age uncertain (Crimea). 



euphyllum, Neiim. Kelloway and Oxford groups. 



— " — ptychoicum, Quenst. Tithonian. 

 semisulcatum, B' Orb. Neocomian. 



5. Isolated types. 



Phylloeeras subobtusum, Kud. Klaus beds. 



viator, IP Orb, Oxford group ? 



Beneckei, Zitt. Tithonian. 



haloricum, Hauer. Klaus beds. 



tortisLilcatum, B' Orb. Klaus beds and Tithonian. 



The first three of these isolated forms, together with P. Rouya- 

 num, D'Orb., from the Neocomian, may perhaps constitute a di- 

 stinct series. 



Each mutation within a series of forms presents very slight but 

 constant differences, without any gradual transitions. The whole 

 of the forms of a series may be considered either to belong to a 

 single species, or to be specifically distinct. The last course seems to 

 be best adapted for geological purposes ; and the genetic connexion 

 may be indicated by adding to the name of the altered form that of 

 th e for m from which it has been derived, under the algebraic symbol 

 V J as proposed by Waagen. 



The objections against these distinctions as representing vari- 

 ties rather than genuine species, may be of weight as regards 

 existing forms, or extinct forms within a given horizon, considered 

 independently of any succeeding or preceding formation ; but in the 

 whole chain of successive organisms the species extends into a series 



